The Digestive Organs of the Alcyonaria and their Relation to the Mesoglœal Cell Plexus

The Dorsal Mesenterial Filaments. The Ventral Mesenterial Filaments. Their histology in starved and well-fed zooids. Their digestive function. Intra-cellular digestion. Discussion of evidence in favour or otherwise of the occurrence of an intercellular digestion in other groups. The presence of an intercellular digestion in Alcyonaria demonstrated by feeding experiments on Alcyonium. Reduction in size of these filaments in tropical forms associated with the increased abundance of zoochlorellæ. Probable symbiosis.

The intimate connection between the plexus and endoderm, and its less intimate connection with ectoderm. The amoeboid character of the so-called nerve-cells and fibrils composing the plexus. Its multiple function.

THE research in connection with this paper is based upon a study of a very comprehensive collection of specimens of Alcyonaria from many localities, now in the possession of the Victoria University of Manchester, and kindly placed at my disposal by Professor Hickson. This includes Mr. J. Stanley-Gardiner’s excellently preserved collections from the Maldive Islands and Funa Futi respectively; Dr. Willey’s collection from New Guinea, New Britain, and Lifu; Professor Haddon’s collection from Torres Straits; Mr. Gilchrist’s collection from the Cape of Good Hope; and Professor Herdman’s collection from Ceylon. I have compared the results of my investigations on these forms with a study of the British representative of the family Alcyonium digitatum in the living as well as the preserved condition.

In two former papers (1903, 1905) I have described the general anatomy and relationships of Alcyonium, Sarco-phytum, Lobophytum, and the new genus Sclero-phytum. The present paper is devoted to a more detailed account of the minute anatomy of the digestive organs, and records an attempted investigation of the physiology of digestion in these genera.

Little is known of the food supply and mode of digestion in the Alcyonaria, and, as the accounts in other groups are very conflicting, experimental evidence has been sought in the hope of obtaining enlightenment as to the nature of the food supply, the physiology of digestion, and the distribution of nutriment in this family. During the spring of 1902 and 1903 numerous feeding experiments were carried out at the biological station of Port Erin on the British Alcyonium digitatum. These yielded very interesting results which are briefly described.¹

I have examined the plexus of mesoglœal cells of Alcyonium, hitherto regarded as a nerve plexus, in the living as well as in the preserved condition, and after a careful comparison with other Alcyonaria, have come to the conclusion that it can be no longer regarded as a differentiated nerve-plexus. We have at the present time no experimental evidence of the existence of a specialised nervous system in the Alcyonaria.

In form, and to a very considerable extent also in structure, a zooid of the monomorphic Alcyonium is very similar to a fully-developed autozooid of the dimorphic Sarcophytum and Lobophytum.

The zooids vary considerably in size. As they are extremely contractile it is frequently impossible to form any true conception of their actual size from preserved specimens. Forms inhabiting tropical waters, however, frequently have smaller and fewer zooids than their relations in temperate seas. The fully expanded living zooids of Alcyonium digitatum are usually very different in form, size, and apparent structure from those in the preserved condition.

A fully-expanded living zooid of Alcyonium digitatum frequently measures from 6 —9 mm. across the crown of tentacles, and the anthocodia often attains a length of from 10—12 mm. The tentacles are from 3—6 mm., and the pinnules about ·5 mm. in length.

In all expanded zooids grooves occur between the bases of the tentacles (fig. 3); they probably serve for the escape of waste fluid and solid matters.

In all the genera the tentacles and pinnules are hollow when expanded. The surface of the latter is dotted with innumerable excrescences, which are due to the presence of batteries of cnidoblasts, each battery in Alcyonium digitatum contains some hundreds of these cells, but they are not usually so numerous in tropical members of the family.

In Alcyonium, Sarcophytum, and Lobophytum the tentacles are fringed laterally with a single row of pinnules. In the genus Sclerophytum the tentacles of some species have one row, and others (Scl. capitale and palmatum) have two, while those of the genus Xenia, according to Ashworth, have three rows of pinnules.

The tentacles and pinnules are apparently shortest in Sclerophytum and longest in Alcyonium, although it is extremely difficult to form an opinion as to their exact size from a study of preserved material alone.

In consequence of the many difficulties which attend the physiological investigation of the food supply and mode of nutrition in this family, little systematic work on this subject has hitherto been attempted. The accounts of the digestive processes in other groups of the cœlentrates are also very conflicting.

Milne-Edwards and Wilson maintain that digestion in the Alcyonaria is intra-cellular, while Hickson states that the food is acted upon by a digestive secretion before it is ingested by the cells lining the digestive tract, i. e. an intercellular digestion occurs in Alcyonaria as well as an intracellular digestion.

In an investigation of the physiological processes of digestion it is necessary that the study of the minute anatomy of digestive organs should be very largely supplemented by observations of the actual digestive processes in living zooids. “Alcyonium digitatum”¹ provides an excellent subject for the study, for not only is-it easily obtainable, but the transparent nature of the body walls of the arthocodiæ enables one to observe many of the stages in the digestion of food within the living zooid, especially if brightly-coloured food material be employed. To obtain a suitable food which could be stained by an innocuous colouring matter was found, however, to be no easy task. It is well known that food material has very seldom been observed in the zooids in collections of preserved material, while in tropical forms it has rarely, if ever, been seen, even in living specimens. In the hope of obtaining some information as to the natural food of the Alcyonaria, some hundreds of autozooids of genera from many localities (including the tropics) were examined, but food was observed only in very few instances in the cœlentera. It then consisted of small partially-digested masses of organic matter, containing fragments of minute Crustacea, zoochlorellæ, and portions of algal filaments, the last-named, however, were apparently unaffected by a digestive ferment.

Several freshly-captured specimens of Alcyonium digitatum were examined, but with the exception of the occurrence of a few fragmentary copepods the cœlentera of the zooids were invariably empty. Certain feeding operations on this form were then attempted, which were at first abortive, but finally successful. These experiments were carried out in the laboratory of the Biological Station at Port Erin where I was successful in keeping colonies of this species for a considerable time under healthy conditions.

Several freshly taken and apparently healthy specimens of Alcyonium were placed in wooden tanks, through which a gentle bub constant stream of filtered sea water was running. Conditions of light and temperature were carefully observed and maintained as nearly normal as possible; the temperature of the water in the tanks never fell more than two degrees below, and never exceeded that of the sea.

Several colonies were also kept in tanks of unfiltered sea water and submitted to the same treatment.

At the end of forty-eight hours most of the zooids of all the colonies were seen with their tentacles completely extended, and were found to be very sensitive to contact.

A fairly well-grown colony was transferred from filtered sea water to a glass vessel containing a concentrated surface tow netting consisting chiefly of Nauplii, small Copepods, Daphnids, and Diatoms. The colony quickly recovered from the transference, and, at the end of half an hour, the tentacles, with their delicate fringes of pinnules, were again extended.

A nauplius, actively swimming so near a tentacle as to lightly brush against the pinnules, was instantly captured by them and paralysed by the innumerable poisoned threads of the nematocysts, and in a very short time the surface of the tentacles was dotted with hundreds of paralysed Nauplii and Copepods. Occasionally a tentacle would curl inwards and deposit its captured prey within the mouth. Usually, however, the zooids, with tentacles outspread, remained expanded for quite an hour, then the colony slowly contracted, and, at the end of a second hour, all the zooids were withdrawn below the surface.

Fifteen hours later the zooids began to slowly expand, and, when expansion was almost complete, the colony was fixed and preserved by a fairly hot 7 per cent, aqueous solution of formalin.¹

After fixing, the colony was submitted to microscopic examination, when minute fragments of Nauplii, of chitinous cases of Copepods and Daphnids were seen to be partially, and in some cases completely extruded from several zooids. Comparatively large specimens of Daphnids and Copepods (Cyclops, etc.) were observed in the cœlentera of several zooids, enfolded and supported by the mesenterial filaments.

The Crustacea which had not been swallowed were found to be excellently preserved by formalin, but the specimens observed in the cœlentera of the zooids were generally found to exhibit unmistakable signs of disintegration. In many cases the empty chitinous shell of a Daphnid still supported by the mesenterial filaments was apparently complete.

I have observed no instance of the ingestion by the filaments of any Copepod, or other fairly large form of prey in a complete state. In many cases the filaments were distended with food material, which, however, was always observed to be in a finely divided condition.

In a second experiment colonies of Alcyonium were confined in running filtered sea water for twenty-four hours. Ripe ova of the flounder were then gently placed upon the extended tentacles, and were immediately enfolded by them. In a few cases only, however, were they swallowed. Usually they were grasped tightly by the tentacles for about a minute and then released. Ova of the plaice, whiting, and cod were substituted with the same result. Believing the ova to be too large to pass through the mouth, extremely small embryos of the crab “Galathea” were offered. These were eagerly taken, enfolded by the tentacles, and afterwards rejected in exactly the same way. The zooids, therefore, exercise considerable choice in the selection of food.

Colonies confined in unfiltered sea water for the same period also refused to feed on fish ova.

An ovum of the plaice which had been swallowed by a zooid was kept under observation for several hours. The comparatively hard shell of the egg remained rounded and apparently intact, but the yolk rapidly became reduced in quantity, and, after five hours, had almost completely disappeared, the egg case being still grasped by the ventral mesenterial filaments. From these experiments it is evident that by some means large food bodies are either broken up into small particles, or at least partially dissolved, in the cœlentera before ingestion by the mesenterial filaments.

In a third feeding experiment a fairly solid homogenous jelly was obtained by pounding the flesh of the whiting. A very small portion of this gently brought in contact with a pinnule was immediately seized, transferred to the mouth by the tentacles, and slowly swallowed. Other colonies which had been kept without food for thirty hours in filtered sea water partook of the food with equal avidity. The pounded flesh of cod, flounder, and plaice were also substituted for whiting, and proved equally acceptable to the zooids.

In order to observe the course of the food within the zooids, the flesh of the fish before pounding was brightly stained by a dilute solution of borax carmine, and afterwards carefully washed in running sea water. The colonies were found to feed with equal avidity on the coloured fish food.

When the colonies were expanded the course of the food could be easily observed through the transparent body wall of the zooids (figs. 2 and 3 illustrate the seizing and swallowing of food, which, on entering the cœlenteron, is grasped and squeezed by the ventral mesenterial filaments, and rapidly disappears). On microscopical examination the food was found to be ingested in such quantity by the filaments as to materially increase their size and to impart to them a red colour (figs. 2—4). The dorsal filaments were observed to take no part in the digestive function.¹

This particular colony was fed five times between 11 a.m. and 7.30 p.m., and was then fixed and preserved in a fairly expanded condition (fig. 1) by the hot formalin method. All the colonies which had been fed with the coloured fish food were found to be in a healthy condition at the end of the experiments, which lasted fourteen days.

Several colonies, kept in tanks of running but unfiltered sea water, were submitted to the same treatment. After three days the anthocodiæ of these colonies were observed to be slightly swollen, and were apparently less sensitive to contact than their neighbours in filtered sea water, and, although expanded, refused to feed. The swollen condition increased; on the sixth day the colonies were fully twice their former size, and were found to be almost insensible to contact. Preserved sections of these colonies were found to be quite useless for histological purposes.

In his account of the Oban Pennatulida, Marshall (1882) remarks upon the swollen condition and loss of sensitive power of specimens in captivity, and states that, as the Pennatulids inhabit deep water, the swollen condition is due to a difference in pressure. All the specimens of Alcyonium which exhibited this condition, however, were taken from shallow water, many of the colonies being exposed at low tide, while three specimens from forty fathoms, and kept in filtered sea water, remained in a normal condition for several days. The swollen and insensible condition in Alcyonium is doubtless pathological, and is quite independent of either increase or decrease in pressure.²

A few colonies confined in filtered sea water for seven days without food were fixed and preserved for comparison with well-fed colonies.

The histology of the mouth disc closely resembles that of the stomodæum (figs. 5 and 6), from which it differs in that the ectoderm is slightly thinner. It differs from that of the tentacles and general ectoderm in the scarcity of nematocysts, and in the presence of numerous granular gland cells similar to those of the stomodæum and mesenterial filaments.

In Alcyonium, and in all other Alcyonaria, with the exception of Xenia, the function of the stomodæum has been believed to be limited to the conveyance of food material to the coelenteron; it has therefore not been considered a portion of the digestive tract.

The stomodæum of zooids, which had been fed just before fixing and preserving, apparently contained no gland cells of any description, but in starved zooids the stomodæum showed a considerable number of gland cells with granular contents identical with cells occurring in the mouth disc and ventral mesenterial filaments (figs. 5 and 6). These cells are very clearly indicated in sections from 2—4μ in thickness, stained with iron brazilin, when the granules become intensely black.

From the presence of gland cells in the stomodæum of starved zooids, and their apparent absence in recently fed zooids, it may be assumed that the food has received a secretion from the cells, in its passage through the stomodæum. (Reference is again made to the subject of secretion in connection with the description of the gland cells of the mesenterial filaments, pp. 342—345.)

In the presence of gland cells in the stomodæum Alcyonium resembles Xenia. The cells, however, differ from those described by Ashworth, 1898, in that genus, in form and in their granular character. In Xenia they are swollen, flask-shaped, and, according to Ashworth, give rise to a mucous secretion. As I have observed both mucous and granular gland cells in the stomodæum of every species of Sarcophytum (fig. 5), Lobophyturn, and Sclerophytum (fig. 6) which I have examined they doubtless occur through-out the family.

Gland cells occur in the stomodæum of several Zoantharia, and have been described in Flabellum by Stanley Gardiner (1902), and in Mæ an dr in a by Duerden (1903). The cells differ slightly in shape and size in the two groups of Cœlen-trates, but doubtless have the same function.

In several members of the Zoantharia the stomodæal ecto-derm is raised into ridges at the insertion of the mesenteries, where in Mæandrina it differs in character from the thinner ectoderm between the ridges. In all the members of the Alcyonaria which I have had the opportunity of examining, the ectoderm of the stomodæum, though usually convoluted in the preserved condition is similar in character and of apparent uniform thickness throughout, no thickenings being apparent at the insertion of the mesenteries.

The stomodæal ectoderm of Alcyonium, Sarcophytum (fig. 5), Lobophytum, and Sclerophytum (fig. 6) is made up histologically of the following elemental cells, which occur in varying proportions, the relative abundance of granular gland cells being in many cases dependent on the condition of the zooids with regard to the supply of food material:

• Granular gland cells, usually irregular in shape, and containing a varying number of rounded granules, which become intensely black when subjected to stains containing iron. These cells are histologically identical with the granular gland cells of the mouth disc and mesenterial filaments. Their function is discussed on pp. 343—345.

• Mucous gland cells with deeply staining nuclei. Each cell contains also a delicate reticulum of protoplasm and a deeply staining mucous secretion,-which occupies the middle and upper portion of the cell, often in the form of a dense deeply staining reticulum. The secretion in some cases may be seen exuding through the outer wall into the stomodæum. In some instances the cells are almost empty. The mucous cells appear to be more numerous in tropical (figs. 5 and 6) forms than in the British species.

• Nematocysts, similar to those occurring in the tèn-tacles, are frequently imbedded in the outer ectoderm.

• Scleroblasts with minute spicules are occasionally observed, but are never numerous.

• Columnar’ cells, usually with a single flagellum, more or less fill up the spaces between the superficial cells.

• Interstitial cells, more deeply seated than any of the above-mentioned cells, are of varying shape, and may give rise to any of the superficial forms of cell.

• Stellate cells, with long processes, are often attached to interstitial and other cells. They often extend into the mesoglœa, and have been described as nerve cells, and the processes as nerve fibrils. They have not, however, been experimentally shown to be nervous in function, but are identical in form, and probably also in function, with certain stellate cells which occur in the mesoglœa (pp. 351—356), and are probably not ectodermic in origin.

The siphonoglyph extends through the entire length of the stomodæum in Alcyonium, Sarcophy turn, Lobophy turn, and Sclerophyturn, and is apparently lined throughout with flagella of uniform size.

The stomodæum of Xenia, Ashworth (1898, p. 443), is described as having a well-marked siphonoglyph, in which only the cells of the lower third bear long flagella. In Lemnalia (Bourne, 1900, p. 532) the siphonoglyph is present as a shallow ciliated gutter, but in some cases entirely disappears, the epithelium of the stomodæum being then ciliated and of the same character throughout. Bourne regards this as a primitive condition from which the siphono-glyph has been derived. In his account of the siphonoglyph in Alcyonaria (1883, p. 699) Hickson states that the tendency of dimorphic forms is to throw the siphonic function upon the siphonizooids, and to eliminate it from the autozooids.

The siphonoglyphs of the autozooids are not so pronounced in the dimorphic Sarcophytum and Lobophytum as in Sclerophytum and the zooids of Alcyonium. In Scl. hirtum the flagella of the siphonoglyph are fully ·03 mm. in length. In the siphonozooids of well-marked dimorphic genera the siphonoglyphs are very large, but are absent in the degenerate siphonozooids of Sclerophytum.

The mesenteries have the same fundamental structure throughout the Alcyonaria. Hickson’s description (1895 and 1900) of the mesenteries of Alcyonium will apply with certain modifications to the zooids of monomorphic, and to the autozooids of dimorphic forms. The ventral mesenteries of the siphonozooids of the latter are extremely small, and even in well-marked cases of dimorphism seldom project beyond the lower end of the stomodæum. In some species of Sclerophytum they are so minute in the siphonozooids as to be almost unrecognisable as such; while in other species of this genus they may be entirely absent (Pratt, 1903, p. 531).

In the autozooids of Sarcophytum the mesenteries are relatively larger than in Lobophytum and Alcyonium. In Sclerophytum they are usually smaller and more feebly developed. The extreme prominence of the mesenteries in Sarcophytum is due to the presence of mesoglœal thickenings near the free edge, which, when examined with low powers of the microscope, have the appearance of enormous mesenterial filaments. The thickenings, however, are due to a localisation of mesoglœal tissue alone (fig. 12), and in cross section have a rounded appearance. The thickenings vary in different species, in “S. ehrenbergi” they are ·05 mm. in diameter, in “S. glaucum” ·03 mm. They occur also in Xenia (Ashworth, 1898), but are much more feebly developed. They probably serve as additional supports to the autozooids when in an expanded condition.

The musculature is typically Alcyonarian, but is much more strongly developed in the dimorphic Sarcophytum and Lobophytum than in Sclerophytum. In Alcyo-nium it is usually well marked, but is more strongly developed in the British species “A., digitatum” than in the tropical “A. pachyclados.” As in Alcyonium the retractor muscles are much larger than the protractors. The pleating of the mesoglœa varies according to the development of the muscles. In Sarcophytum, Lobophytum, and Alcyo-nium the folds are numerous and very prominent, in Sclerophytum they are smaller and vary in size in different species. In the species “Scl. palmaturn” and “Sel. capitale” they are not numerous, but are fairly large, but in the species “Scl. poly dactylum “and “Scl. gardineri” the folds are very few and extremely small. The musculature of the mesenteries is more strongly developed in the upper than in the lower portions of the zooids. In the genus Sclerophytum it seldom extends below the terminal portion of the stomodæum. The musculature of the mesenteries of the siphonozooids is always feebly developed, as these individuals are only very slightly contractile. In the siphonozooids of Sclerophytum it is entirely absent.

In the Alcyonaria the stomodæum is continuous with the mesenteries. Of the three layers which compose the stomodæum, only the endoderm and mesoglœa are continuous with the ventral mesenteries. There can be no doubt of the termination of the ectodermic epithelium of the stomodæum at its aboral opening (fig. 8). Further reference is made to this fact (pp. 341 and 342).

E. B. Wilson (1884, p. 12) has shown that the dorsal mesenterial filaments have almost precisely the same structure throughout the Alcyonaria. As they are fully described, and their ectodermic origin established by him, it is necessary to add but little to his excellent account of these structures.

Throughout the family these filaments are very long. In the siphonozooids of well-marked dimorphic genera they are proportionately very much longer and more strongly marked than the ventral filaments, and are proportionately less pronounced in the autozooids.

In his account of the mesenterial filaments of the Alcyonaria E. B. Wilson (1884, p. 22) states:

“There can be no doubt that the compound Alcyonaria are derived from solitary forms, which probably possessed eight similar filaments, each consisting of an ectodermic circulatory part and an entodermic digestive part. As the colony-forming habit became established, bringing with it the need for specialised organs of circulation, a physiological division of labour took place among the filaments. In the dorsal pair the ectodermic part gradually supplanted the entodermic, while the reverse process took place in the other six.” He further states that we have no embryological evidence of this, but suggests that the portion of the ectoderm (ect.) of the stomodæum which is in immediate continuity with the ventral mesenteries probably represents the original ectodermic part of the ventral filament.

A study of vertical sections of the stomodæum shows (fig. 8) this ectodermal tissue (ect.) to be the ectoderm of the lower end of the stomodæum which has become fused with the mesenteries. Between the mesenteries the ectoderm becomes thinner towards the free edge, and is identical in every sense with that portion which has fused with the mesenteries.

We have, therefore, no evidence in favour of Wilson’s hypothesis of an ancestral identity of form, origin, and multiple function of the dorsal and ventral mesenterial filaments.

The ventral mesenterial filaments of the Alcyonaria exhibit considerably more variety in form, size, and, to a certain extent, in structure than do the dorsal filaments.

In his account of the anatomy of Cœnopsammia Stanley Gardiner (1900) maintains that the mesenterial filaments, together with the stomodæum, are ectodermic in origin in that form, and says:

“The stomodæum of the Zoantharia, and necessarily also of Alcyonaria, is not comparable to the stomodæum of the Triploblastica, but rather is, with the mesenterial filaments, the homologue of the whole gut. The so-called endoderm, giving rise to the muscular bands and generative organs, and performing also the excretory functions, is then homologous with the mesoderm of Triploblastica. In the terms of the layer theory, of whatever value it may be, the Actinozoon polyp must then be regarded as also a Triploblastic form having ectoderm, endoderm, and mesoderm.”

E. B. Wilson (1884, p. 7) states in the development of Funiculina that the ventral mesenterial filaments arise quite independently of the stomodæum, and are endodermic in origin. What Wilson has shown for Funiculina may be true of other Alcyonaria. I have already shown (fig. 8) that the ectoderm terminates with the aboral opening of the stomodæum in the adult condition, and only the mesoglœal and endodermal tissues are continued downwards into the mesenteries. Yet a histological study of the mouth disc, stomodæum, and ventral mesenterial filaments in several members of the family reveals many points of similarity, if not identity, in their elemental constitution. Both granular and mucous gland cells, as well as nematocysts, occur in all these structures.

Milne-Edwards in 1835 was the first to attribute a digestive function to the mesenterial filaments of the Alcyonaria. The presence of gland cells in the ventral mesenterial filaments of Paralcyonium was first observed by Wilson, 1884, who states that they are similar to those observed by the Hertwigs in the Actinians. He also describes the occurrence of ingested foreign bodies in the filaments.

Hickson (1895, p. 367) described two kinds of gland cells in the ventral filaments of Alcyonium digitatum, one kind being large, unciliated, and deeply staining with hæma-toxylin, the other consisting of elongated columnar cells filled with numerous minute granules. He further states (1901, p. 12) that the function of the ventral filaments is to secrete a digestive juice upon particles of food which have passed through the stomodæum.

J. Stanley Gardiner (1900 and 1902) describes the occurrence of granular and mucous cells in the mesenterial filaments of the Madreporaria, and in his description of Flabellum says:—“ Every stage of ingestion and protrusion of foreign matter could be seen in the swollen-out endodermal bases of the mesenterial filaments, but elsewhere was not observed. The storing up of round, fat globules, not only in the endoderm at the bases of the mesenterial filaments, but anywhere in the endoderm, indicates that there must be a true digestion—due to the secretion of the gland cells of the mesenterial filaments—and absorption over the whole endo-derm, as well as ingestion at the bases of the filaments. No absorption would, however, seem to occur in the mesenterial filaments, the concentration of fat, etc., in the endoderm at their bases being correlated with this.”

The Zoantharia are well known to be widely separated genetically from the Alcyonaria. Nevertheless, the record of the secretion of a digestive juice is of great importance, for in a recent publication Mesnil (1902) states that digestion in the Actinians is entirely intra-cellular, and denies the occurrence of an inter-cellular digestion in the group. This statement is based on the result of a number of experiments of a chemico-biological character.

On comparing sections of the ventral filaments of recently-fed zooids with similar sections of starved zooids, a considerable amount of histological difference was observed (figs. 9—11).

The filaments of starved zooids were densely crowded with gland cells containing numerous rounded granules, which became so intensely black on staining with iron brazilin and iron bæmatoxylin, that their histological structure could only be observed in very thin sections (3—5µ). Each gland cell was then seen to contain a deeply-seated nucleus and a delicate reticulum of protoplasm, in which the granules are imbedded. The gland cells near the surface of the filament usually contain more granules than the younger more deeply-seated cells (fig. 10). It is worthy of note that the granules and ingested food matter have not been observed together in the same cell.

Gland cells identical in structure, aud, doubtless, also in function, have also been observed in the stomodæum and mouth disc.

A few mucous cells are interspersed between the granular gland cells and amoeboid endoderm cells.

Sections from 15—20 μ in thickness were cut through the filaments, distended with carmined fish food (fig. 4), and microscopically examined without staining. The food was observed to be ingested in an amoeboid manner by the endoderm cells covering the filaments (fig. 9). Particles of food were observed in the act of ingestion (f. f.), and particles of waste matter were also seen to be extruded from the cells (f.u.).

Within the cells the food material quickly became enveloped in food vacuoles, and speedily disintegrated; the red colour disappeared, and the process of digestion was apparently completed.

Similar sections were also stained with iron brazilin and examined in the same way. These were found to contain numerous gland cells, which were either empty or contained only a few granules (fig. 11).

From the feeding experiments the following facts are gleaned, which have an important bearing on the question of the occurrence of an inter-cellular digestion in Alcyonium:

1. Large food bodies are rapidly broken up into small particles, and in some cases apparently acted upon by some digestive ferment in the cœlentera of the zooids before they are ingested by the ventral mesenterial filaments.

2. The mesenterial filaments of hungry zooids are crowded with gland cells containing numerous granules.

3. These gland cells also occur in the stomodæum and mouth disc of hungry zooids.

4. The mesenterial filaments of zooids immediately after feeding contain very few granular gland cells in which the granules are numerous, many cells contain very few granules, and several gland cells are empty.

5. The stomodæum and mouth disc of zooids immediately after feeding are usually devoid of granular gland cells.

The only inference to be drawn from a consideration of these facts is that the gland cells of recently-fed zooids have poured on to the food, during its passage through the stomodæum and envelopment by the filaments, a digestive secretion, which has brought about its disintegration and partial solution before its ingestion by the mesenterial filaments. Therefore, we have evidence in the Alcyonaria, as in the Madreporaria, of an inter-cellular digestion by the secretion of a digestive fluid into the cœlentera of the zooids, as well as an intra-cellular digestion which occurs throughout the Coelenterates.

I have already drawn attention to the fact that food material is seldom observed in the cœlentera of the zooids in Alcyonaria (p. 331). This is especially the case with regard to tropical forms, and has been commented upon by several authors (p. 347).

Histologically, the stomodæum and ventral mesenterial filaments differ to a greater or less degree from those of the British Alcyonium digitatum. Several tropical species, Sarcophytum glaucum, Sclerophytum capitale, palmatum, densum, etc., give rise to a copious mucous secretion. In these forms mucous gland cells are extremely numerous in the stomodæum (figs. 5 and 6).

In tropical forms the mesenterial filaments are frequently small compared with those of colonies inhabiting temperate waters. This is particularly noteworthy in the tropical specimens of Alcyonium pachyclados. In specimens from the Cape which have been attributed to this species the mesenterial filaments are fairly well developed, but in colonies from the Maidive Islands they are either extremely small or entirely absent. The enormous size of the filaments in Sarcophyturn I have shown to be entirely due to the thickening of the mesoglcea of the mesentery near the free edge (fig. 12), and cannot in any sense be regarded as an increase of digestive surface. In many cases of this genus the mesenterial filaments are small compared with those of the British genus (figs. 10 and 12), and contain few granular gland cells.¹

The filaments are apparently larger in Sarcophytum glaucum than in any other species, but it is to be regretted that several specimens are not sufficiently well preserved for the study of the histology of the filaments; when gland cells are present they are similar to those of Alcyonium (fig. 10), and doubtless fulfil the same function.

The mesenterial filaments of Sarcophytum latum closely resemble those of Lobophytum, but as this species resembles Lobophytum in several other respects (Pratt, 1903) it should henceforth be included in this genus.

These filaments in Lobophytum are more like those of the British form than any other tropical genus in the collection. They are, however, smaller than in our species of Alcyonium, and it is interesting to note that zoochlorellæ are by no means numerous.

The ventral filaments of Sclerophytum vary considerably in different species (cf. Table, Pratt, 1903, p. 531). They are smaller than in Lobophytum, and in some cases their presence is extremely doubtful.¹ They differ from those of Sarcophytum and Lobophytum in that they are frequently crowded with zoochlorellas, and from the latter genus also in that granular gland cells are very scantily distributed (fig. 13, Scl. capitale). Fragments of zoochlorellas sometimes occur in the filaments, and there is little doubt that these cells are digested by the zooids. I have examined several specimens of the species of this genus, and have been unable to find other food material in the cœlentera, or in an ingested condition in the mesenterial filaments. The scarcity or complete absence of food material in the cœlentera of tropical corals is well known (p. 331), and has been commented upon by Hickson for Hydro cor allin es, by Hickson, Bourne, Fowler, and Duerden for Madreporaria. Brandt and Hickson suggest that zoochlorellæ contribute nutriment in a state of solution to the corals in a mature condition.

After experimenting on Radiolaria and Anemones Famintzin (1891) maintains that these cells can only afford nutriment to the animals by the actual digestion of their tissues.

Gamble and Keeble (1903) have experimented on Convelida roscoffensis, a Turbellarian which contains green cells in great abundance. They find that Convoluta feeds voraciously from the time of hatching to the period of maturity when it adopts a new mode of nutrition, and “derives all its food directly from the green cells by digesting them, and possibly also indirectly by extricating plastic nutriment from them.” Dr. Gamble informs me that since the publication of this paper he has obtained evidence in favour of the last supposition.

The foregoing comparative description of the histology of the genera Alcyonium, Lobophytum, and Sclerophy-tum indicates a reduction of the digestive surface of the autozooids in tropical forms associated with a corresponding increase in number of zoochlorellæ,¹ and may be summarised as follows:

1. The ventral mesenterial filaments of Lobophytum more closely resemble those of the British Alcyonium, and are only slightly reduced. Food material has been observed in the cœlentera of this genus. Zoochlorellæ are never numerous.

2. In Sarcophytum the mesenteries are modified by mesoglœal thickening near the free edge, but the filaments are smaller than those of Lobophytum, and are provided with few gland cells. Zoochlorellæ are fairly numerous.

3. The filaments of the tropical species of Alcyonium are extremely small, and contain few gland cells. Food material was not observed. Zoochlorellæ are very numerous.

4. The ventral mesenterial filaments in Sclerophytum are either very small or entirely absent.² When present, gland cells are so few in number that their physiological function must be extremely limited (fig. 13). No foreign food material was observed. Zoochlorellæ are extremely numerous.

From the comparatively small number of zooids in Sclerophytum and the minute size of the tentacles it is obvious that the amount of food captured by the latter must be extremely small and totally inadequate to supply the growing needs of a colony. Furthermore the minute mesenterial filaments—(the degenerate representatives of the principal organs of digestion in the British Alcyonium)—and the stomodæum are together incapable of digesting a sufficient amount of food to serve for the nutrition of an entire colony.

I have already experimentally shown (p. 332) that the natural food of the British Alcyonium appears to consist chiefly of small living Crustacea, which, captured by the long and extremely contractile tentacles, are paralysed by the poisoned threads of innumerable nematocysts before being swallowed. The absence of food in many tropical Alcyonaria may be attributed to the degeneration of the zooids, which appear to have not only lost the power of capturing living prey but also of killing and digesting it.

I have already shown that zoochlorellæ are most numerous in the genus Sclerophytum, in which the reduction of the digestive surface reaches its extreme limit. It therefore seems very possible that these algal cells indirectly contribute nutriment in a soluble condition to the corals they inhabit, as well as directly by their actual digestion. This matter is further’ discussed in the following portion of the paper devoted to the description of the structure and function of the zoochlorellæ.

I have already described the occurrence and relative abundance of these algal cells in Sarcophytum, Lobo-phytum, and Sclerophytum (1903), and have drawn attention to the fact that while they usually occur in colonies inhabiting shallow water they are found to be fairly abundant in specimens from 24—34 fathoms, so that their numbers do not appear to be affected by bathymetric variations within certain limits. I have suggested that their presence in enormous numbers in the superficial tissues in certain species of Sclerophytum is correlated with a reduction in size of the tentacles and mesenterial filaments.

The geographical distribution of zoochlorellæ is interesting. These algal cells commonly occur in tropical Alcyonaria, but are usually absent in corals inhabiting the temperate and cold waters of British seas and the South Atlantic (Cape of Good Hope). Hickson (1894), however, describes them as being extremely abundant in species of Clavularia from the Victorian Coast of Australia.

In a preceding part of the present paper I have dis-cussed the more or less gradual reduction of the digestive surface in tropical members of the Alcyonaria, and the accompanied increased abundance of zoochlorellæ in connection with the scarcity or absence of food in these forms.

As these cells are frequently observed in a partially-digested condition, they no doubt serve as a direct source of nutriment to the corals they inhabit.

The zoochlorellæ of the Alcyonaria are very similar to those described by Duerden (1903) in the Madreporaria, and do not apparently differ in any essential respect from those inhabiting other tropical corals. A cell usually has one, but many have two chromatophores. The presence of starch in these algal cells may be easily demonstrated by treating with a dilute solution of caustic potashfollowed by iodine solution. A starch ring, sometimes incomplete, is then seen to surround the pyrenoid, and in many cases starch grains are scattered about the middle of the cell. The presence of reserve food material in the form of starch in these algal cells indicates a super-abundance of nutriment. This insoluble food material can only be converted into a soluble form, such as sugar, by the action of diastase secreted by the protoplasm of the alga or possibly also by the animal cell which it inhabits. ¹ It is well known to botanists that the vegetative cells of plants may convey nutriment in soluble form from one cell to another. In lichens nutriment of a carbohydrate, and possibly also of a nitrogenous nature is prepared by the algal cells and is conveyed to the symbiotic fungus through the walls of the algal cells and fungal hyphæ.

Zoochlorellæ occur only in those portions of the corals which are exposed to light, and are most abundant in the endo-dermal cells and spaces, continually bathed with sea water, which circulates more or less freely within the colony through the zooids and canals. It is obvious that the circulating sea water rapidly becomes charged to a generous extent with carbon dioxide and other products of animal metabolism. The presence of carbon dioxide in considerable proportion would enable the rapid formation of carbohydrate food-material in the algal cell under the influence of sunlight. These cells have also the power to build up organic nitrogenous compounds from the inorganic nitrogen salts contained in the sea water, but they may also make use of the waste nitrogenous animal matter, in which case the alga too would derive some benefit from the symbiosis.

In the reduction in size and function, or complete loss of the organs of digestion in corals greatly infested with zoochlorellæ, we have evidence that these algal cells nourish the coral to a considerable extent by contributing carbohydrate, and possibly also nitrogenous food material in a soluble condition.

The cells and fibrils which compose the so called “mesoglœal nerve plexus” of the Alcyonaria were observed to be extremely numerous in some members of the family and comparatively rare in others, while in some instances they appeared to be of an unusually large size.

In his account of the anatomy of the Alcyonaria, Hickson (1895, p. 371) calls attention to the fact that, while this system of cells and fibrils has not been experimentally shown to be nervous in function, yet it is undoubtedly homologous with the “nervenschicht ”described by the Hertwigs in the Actineæ (1879). Ashworth (1898, p. 209) describes and gives admirable figures of a nervous plexus in Xenia which he states to be homologous with that of Alcyonium.

Kassianow, 1903, gives a preliminary account of the nervous plexus of “Alcyonium.”

In preserved specimens of Sarcophytum, Lobophytum, Sclerophytum, and Alcyonium the cells and fibrils vary considerably in size, shape, and relative abundance. In certain specimens the cells have a stellate form, and are provided with numerous long and short fibril-like processes (fig. 15, Sclerophytum durum). In some forms they are polygonal, with fewer processes (fig. 16, Lobophytum pauciflorum), while in others they are less numerous, are somewhat spindle shaped, and have only two or three processes (fig. 17, Sclerophytum densum). The fibril-like processes usually have a hyaline structure, are sometimes very long, and frequently fuse with each other, so as to form a more or less complete network (figs. 18 and 21), which is known as the mesoglœal nerve plexus. Where fusion has taken place the processes have a granular protoplasmic appearance similar to the cell contents.

Many of the cells are intimately connected with the endoderm cells of the zooids and canals¹ (figs. 17 and 18), but the connection between the cells and the ectoderm is less intimate (fig. 16). Occasionally a cell may be connected by means of its processes, with an ectoderm cell on one side, and an endoderm cell on the other.

These cells, with their long fibril-like connections with ectoderm and endoderm, present, in the preserved condition, a remarkable likeness to nerve cells and fibrils occurring in other groups, but, beyond this resemblance, we have no evidence of their nervous character; moreover, the very intimate connection existing between the plexus and the endoderm (fig. 18), and its less intimate connection with the peripheral ectodermal tissues (fig. 16), throws considerable doubt upon the theory of its having a special nervous function.

In order to ascertain, therefore, its true nature and function, I examined the plexus in living specimens. A comparison of preserved preparations with the living plexus of cells in Alcyonium digitatum yielded results which are as interesting as they were unexpected.

Thin free-hand sections of Alcyonium were examined in the living condition with moderately high powers,³ when the cells of the “mesogloeal plexus” could be observed without difficulty. The stellate cells were then seen to with-draw and thrust out the processes which have been called nerve fibres. Several cells were sketched with the aid of a camera lucida at intervals of from twenty minutes to half an hour, and in every case they were observed to be in an amoeboid condition, the psendopodial processes being more or less numerous, long and slender in form, and frequently branched (figs. 19 and 20).

The rapidity with which the cells change their outline varies considerably. The cell shown in fig. 19 was moving much more quickly than that of fig. 20. The so-called nerve fibres are simply the pseudopodia of the amoeboid cells, which vary in size according to the cell’s mode and rate of progression. The curious stellate appearance of the cells in the preserved condition (fig. 15) is doubtless due to their contraction on fixing.

Acting on the advice of Professor Hickson, I carried out the following experiments:

Minute particles of carmine were suspended in the sea-water in which living colonies of Alcyonium were kept. For three days clouds of carmine were squirted, by means of a pipette, about the expanded zooids. Thin free-hand sections were then cut, and, on examination, minute particles of carmine were observed in an ingested condition in the endoderm cells of the ventral mesenterial filaments and in the endoderm of the body walls of the zooids.

It is interesting to note that the ingestion of foreign particles of carmine is amoeboid, and is identical in every respect with the ingestion of food material (fig. 9).

The experiment was continued for seven days. After the fourth day carmine particles were observed, first in the cells of the endoderm canals, and then in the cells of the solid cords of endoderm in the mesoglcea. In both instances some of the cells containing carmine particles were seen to be in an amoeboid condition (fig. 22a and 22b) and to thrust out processes into the mesoglcea.

Finally, particles of carmine were also seen in the stellate and spindle-shaped amœboid cells forming the so-called “mesoglœal nerve plexus “(fig. 24). Some of these cells were kept under observation for a considerable time. They frequently remained in an active amœboid condition for quite an hour, then the pseudopodia would be withdrawn, and the cells would become rounded and inert. Such a condition is comparable with the rounded cells frequently observed in the mesoglœa of stained preparations.

These experiments substantiate the following facts:

1. Solid particles of carmine are ingested by the endoderm cells of the mesenterial filaments, body wall, and endodermal canals (figs. 21, 22, and 24) in a manner precisely similar to the ingestion of food particles by the mesenterial filaments (fig-9).

2. The endoderm cells of the ventral mesenterial filaments, the body wall, the canals, and cords in the mesoglœa are frequently amoeboid. ‘

3. The presence of ingested carmine particles in the cells of the mesoglœal plexus indicates that they have been conveyed from the cœlenteric cavities of the zooids to portions of the colony apart, or even remote, from the zooid.

4. The cells composing the so-called “mesoglœal nerve plexus” are amœboid, and the so-called “nerve fibres” are really the pseudopodia of the amoeboid cells.

5. The mesoglœal plexus is more intimately connected with the endodermal than with the peripheral ectodermal tissues, while the cells, apparently of the same nature as the endoderm, are frequently larger in the neighbourhood of the endoderm than near the ectoderm.

These facts afford evidence that the so-called “nerve cells and fibres” are really amoeboid endoderm cells which wandered into the mesoglœa, forming with their pseudo-podial processes the more or less dense protoplastic mesh-work known as the “mesoglœal nerve plexus.”

Although it is well known that, in the embryonic stages of higher forms of life, ganglion cells have a certain power of movement through the tissues, yet we have no reason for believing that nerve cells retain this power when maturity is reached. Amoeboid nerve cells and pseudopodial nerve fibres are unknown. We have no evidence that the Alcyonaria are more nervously sensitive than other lowly organised groups, so that it is impossible to regard this extremely well-developed system of amoeboid cells with coalescing pseudo-podia as a specially differentiated “nervous plexus.”

1. Functions of the Mesoglœal Plexus.—The ingestion of the inorganic particles of carmine by the endoderm cells is identical with the ingestion of organic food material. The distribution of ingested foreign matter by means of the wandering amœboid cells, usually most abundant about the digestive centres, is no doubt similar to the distribution of food material in a digested condition. I would, therefore, suggest that the distribution of nutriment is effected in the following manner:—Certain amœboid endoderm cells loaded with nutriment wander, or have wandered, into the mesoglcea, where they form an amœboid plexus of cords and strands of cells which extends throughout the colony. The intimate connection between the digestive endoderm cells of the zooids and the plexus is maintained. If we suppose that throughout the plexus the nutritive protoplasm may be transferred from cell to cell—and the presence of carmine particles in the mesoglœal plexus affords substantial evi-dence for believing this to be the case—then this system of amœboid cells must be regarded as a nutritive as well as a sensitive plexus, and by its means nutriment may be conveyed from the digestive endoderm cells of the zooids to every portion of the colony.

2. Excretory Function.—As the amœboid endoderm cells were observed to eject foreign bodies in the form of carmine particles (fig. 9, f. u.), it is very possible that the plexus has also an excretory function, the amœboid cells, which are to be regarded as the carriers of nutriment may also convey waste products to the cœlentera or lumen of the canals,

3. Nervous Function.—As a stimulus affecting one polyp may be transmitted with gradually diminishing effect to its neighbours, it is probable that stimuli or impulses travel through this unspecialised amœboid protoplasmic plexus. As the speed of transmission might possibly be retarded by the presence of nutrient matter in the proto-plasm, the colonies would therefore become less sensitive to stimuli during the distribution of the digested food material. This may explain the fact that the colonies under experiment (p. 332) withdrew their anthocodiæ shortly after feeding, and remained in a contracted condition for several hours.

In their amœboid character and multiple function these cells are homologous to the phagocytes occurring in other groups.

The apparent lack of differentiation in their structure and function must be considered a secondary feature. Certain cells, at one time forming a constitutional part of the endoderm, have reverted to a more primitive amœboid condition, in which they are capable of fulfilling any function which the demands of the colony may require them to perform.

The research in connection with this paper has been carried out in the Zoological Laboratories of the Victoria University of Manchester, in the Biological Laboratory of Port Erin, and in the Zoological Laboratory at Naples.

I am greatly indebted to Professor Hickson for much valuable advice and kind supervision of my work.

Illustrating Edith M. Pratt’s paper, “The Digestive Organs of the Alcyonaria and their Relation to the Mesoglœal Cell Plexus?’

Amb. c. Amœboid cells, b. cn. Batteries of cnidoblasts, car. p. Carmine particles, c.g.c. Clear gland cells, ch. Chromatophore. cn. Cnidoblast. cœl. Coelenteron, col. end. Columnar endoderm, c. v. Ciliated vessel, c. z. Contracted zooids, d. mg. Dense layer of mesoglœa surrounding zooids. d. mes. Dorsal mesentery, d. m.f. Dorsal mesenterial filament, ect. Ectoderm. em. g. c. Empty gland cell. end. Endoderm, end. b. w. Endoderm body wall. end. can. Endodermal canals in mesoglœa. end. co. Endodermal cords in mesoglœa. f. c. Flagellate cel), f.f. Carmined fish food. f. u. Partides of extruded undigested food. f. v. Food vacuoles, g. c. Golden cells. gr.c. Granular gland cells, in.mes.sp. Intermesenterial space. int.c. Interstitial cell. ini. c. s. Internal canal system. I. c. Longitudinal canals, I. m.f. Lateral mesenterial filament, m. Mesentery, m. ap. Mouth aperture. mg. Mesoglcea. mg. b. w. Mesoglcea body wall. muc. c. Mucous gland cell. nem. Nematocyst, nu. Nucleus, nue. Nucleolus. ov. Ovum. v. mes. Mesenteries coloured red by ingested fish food. s. Stomodæum. sc. Scleroblast, si. Siphonozooid, sp. Spicule, sph. Hole left by spicule after decalcification, sta. Starch, si. c. Stellate cell = amœboid cell. tent. Tentacle, is. s. c. Transverse superficial canal, v. m. Ventral mesentery. v. m.f. Ventral mesenterial filament, zo. Zoo-chlorellæ.

PLATE 20.

FIG. 1.—Alcyonium digitatum. A fairly young colony which has not yet assumed the digitate form. The zooids have been fed on carmined fish food, which can be seen through their transparent body walls. The colony was fixed and preserved in a fairly expanded condition by the hot formalin method, ⁠.

FIG. 2.—Alcyonium digitatum. Diagrammatic representation of the capture and course of food within the zooid.

a. Capturing food.

b. Tentacles contract slightly and enclose food.

c. Swallowing food.

(The next stage—the grasping and squeezing of food by the ventral mesenterial filaments—is shown in Fig. 3.)

d. The ventral mesenterial filaments are coloured red by ingested particles of carmined fish food.

The dorsal mesenterial filaments take no part in the digestive process. × 10 (cam. lue.).

PLATE 21.

FIG. 3.—Alcyonium digitatum. Anthocodia of azooid which has been fed on carmined fish food. The excrescences on the pinnules of the tentacles are batteries of nematocysts. Grooves are shown between the bullate bases of the tentacles. The ventral filaments are shown embracing the food as it emerges from the stomodæum. × 10 (cam. lue.).

FIG. 4.—Alcyonium digitatum. A mesenterial filament of a zooid which has been fed for two days on carmined fish food. The red patches are ingested particles of coloured food, × 67 (cam. lue.).

FIG. 5.—Sarcophytum glaucum. Transverse section through stomo-dæal ectoderm to show granular and mucous cells. × 620 (cam. lue.).

FIG. 6.—Sclerophytum densum. Transverse section through stomo-dæal ectoderm to show mucous gland cells. The mucous secretion is seen to be oozing from many of the cells. × 608 (cam. lue.).

FIG. 7.—Sclerophytum hirtum. Transverse section through the siphonoglyph, showing the extremely long flagella, which are about ‘03 mm; in length. × 608 (cam. lue.).

FIG. 8.—Lobophytum pauciflorum. Longitudinal section through the lower portion of the stomodæum of an autozooid, showing the termination of ectoderm and the continuation of endoderm into the ventral mesenterial filaments. × 313 (cam. lue.).

FIG 9.—Alcyonium digitatum. Transverse section through an un-stained ventral mesenterial filament, showing amœboid ingestion of carmined fish food by the endoderm cells. Some of the amœboid cells are shown with pseudopodia projecting into the cœlenteric cavity. Particles of undigested food (f. a.) are being extruded from some of the cells. Gland-cells are not indicated, as they cannot be seen without the aid of staining reagents. × 930 (cam. lue.).

FIG. 10.—Alcyonium digitatum. Transverse section through a ventral mesenterial filament of a starved zooid, stained with iron brazilin. Granular gland cells (gr. c.) are extremely numerous. A few mucous cells are interspersed between the granular gland cells and the amœboid endoderm cells. There are no spaces between the gland cells at the periphery of the filament, but spaces are numerous in the middle of the filament. × 930 (cam. lue.).

FIG. 11.—Alcyonium digitatum. Slightly oblique section through a ventral mesentery of a recently fed zooid, similar to the one shown in Fig. 10, after staining with iron brazilin. This filament differs from that of a starved zooid (Fig. 11) in that the granular gland cells are remarkably few in number andcontain very few granules. The spaces occurring between the cells at the edge of the filament are probably empty gland cells which have discharged their secretion on to the food when it was embraced by the filaments. A few nematocysts, similar to those of the tentacles, are present. × 930 (cam. lue.).

FIG. 12.—Sarcophytum ehrenbergi. Transverse section through a ventral mesentery to show the mesoglœal thickening near the free end, and the scarcity of gland cells in the feebly marked filament. × 416 (cam. lue.).

FIG. 13.—Sclerophytum capitale. Slightly oblique transverse section through a ventral mesenterial filament in which zoochlorellæ are extremely numerous and granular gland cells very few in number. This drawing illustrates the reduction of the digestive surface in tropical forms, and the corresponding increased abundance of zoochlorellæ. × 930 (cam. lue.).

FIG. 14.—Lobophytum pauciflorum. Slightly oblique transverse section through a ventral filament. This is a tropical form which contains comparatively few zoochlorellæ. The filament is fairly large, and has fairly numerous granular gland cells. (Compare fig. 10.) This is the only tropical species in which I have observed the presence of food material, × 416 (cam. lue.).

PLATE 22.

FIG. 15.—Sclerophytum durum. Stellate cells with fibril-like processes, which compose the mesogloeal “cell plexus,” from a stained preparation. × 826 (cam. lue.).

FIG. 16.—Lobophytum pauciflorum. Transverse section through the wall of an autozooid at the base of a tentacle to show the processes from the inner ends of the ectodermal cells, and their connection, in some cases, with the stellate cells of the mesogloeal plexus, which have fewer processes than in fig. 18. × 826 (cam. lue.).

FIG. 17.—Sclerophytum densum. Section through the terminal portion of an endodermal canal in the mesoglcea showing its intimate connection with the cells of the mesogloeal plexus. Many of the cells are more or less spindle-shaped, and have very few processes. (Compare with figs. 15 and 16.) × 826 (cam. lue.).

FIG. 18.—Alcyonium digitatum. Drawing showing the intimate connection between the stellate cells of the plexus and the endodermal canals in a living colony. × 706 (cam. lue.)..

FIG. 19.—Alcyonium digitatum. Two drawings of a living amœboid cell of the mesogloeal plexus. Half an hour elapsed between the drawings. The cell is moving in an upward direction, and changes its outline more rapidly than in fig. 20. a × 930, c × 960 (cam. lue.).

FIG. 20.—Alcyonium digitatum. Three drawings of a single living amœboid cell with long pseudopodia of the mesogloeal plexus. (Twenty minutes elapsed between a and b, and thirty minutes between b and c) The pseudopodia (so-called “nerve fibres”) of the lower part of the cell are being withdrawn while long new pseudopodia are being thrust out from the upper part. The cell is obviously moving in an upward direction, × 723 (cam, lue.).

FIG. 21.—Alcyonium digitatum. Diagrammatic representation of the course of carmine particles suspended in the sea-water in which living colonies were confined. After two days these particles were observed to be ingested by the endoderm cells of the ventral mesenterial filaments and the endoderm lining the body-walls, and after from four to seven days were observed in the cells of the endoderm of the canals and cords in the mesoglcea, and finally in the amœboid cells which bave hitherto been regarded as nerve cells.

FIG. 22.—Alcyonium digitatum. a. A portion of an endodermal cord in the mesoglœa in a living specimen. The cells contain ingested carmine particles. b. The same cord after an interval of half an hour. Two of the cells containing carmine are in an amœboid condition, and are beginning to wander into the mesoglœa. × 547 (cam. lue.).

FIG. 23.—Sclerophytum densum. a. Group of endodermal canal cells, one of which is in an amœboid condition, and has an extremely long, branched pseudopodium thrust into the mesoglœa. (Compare with fig. 25.) × 800 (cam. lue.). b and e are amœboid endoderm cells which have wandered into mesoglœa. × 800 (cam. lue.).

All from stained preparations.

FIG. 24.—Alcyonium digitatum. Living amœboid cells of mesoglœal plexus containing particles of carmine. These have hitherto been known as nerve cells, × 717 (cam. lue.).