No observation made within the last three years appears to me to have greater importance and general significance for the progress of Biology than the discovery of the inception of solid food particles by endoderm cells in the Planarians and Nematophorous Cœlentera, by Elias Metschnikoff.
The actual history of this discovery appears to date from the observations of Lieberkuhn onSpongilla(‘Müller’s Archiv,’1857). The first observer to suggest the existence of intra-cellular digestion in an organism other than one of the Protozoa or of the Porifera, was Allman, who, in his memoir on Myriothela (‘Philosoph. Transact./ vol. 165,1875, p. 552), describes a thin layer of protoplasm as occurring on the free surface of the endoderm, and observes that “its occurrence, with its pseudopodial extensions, on the gastric surface of the animal, is full of interest, and suggests a close analogy between the absorptive action of the gastric surface and amoeboid reception of nutriment”
Next we have a note by Metschnikoff in the ‘Zoolog. Anzeiger,’ 1878, p. 387, in which the inception of solid food particles by the cells lining the alimentary canal of certain Planarians is described, and in the ‘Zeitsch. wiss. Zoologie 1879, p. 371, the same author describes similar observations on Sponges.
Led by these observations of Metschnikoff, Jeffery Parker came to the conclusion that a similar mode of digestion obtains in Hydra. In his paper on the histology of Hydra fusca, published in the ‘Proceedings of the Boyal Society‘1880, and in this Journal, April, 1880, Parker carefully describes the amoeboid character of the endoderm cells of Hydra as seen in sections, the extent and activity of their movements during life having been previously insufficiently recognised. Dark-coloured irregular granules of various sizes are found within these cells, and were noted by Kleinenberg to vary in number with the state of nutrition of the animal. Parker is convinced that these bodies are food particles, taken into the protoplasm of the cells, from the partially disintegrated bodies of the Entomostraca in the digestive cavity. The clearest case of ingestion of solid particles observed by Parker was when a diatom was seen to be completely embedded in the protoplasm of a cell. Parker very judiciously observes that it is quite possible that a preliminary disintegration of the animals taken in is performed by juices secreted by the endoderm cells; but the final digestion seems to take place in the actual protoplasm of the cells, into which the food particles are taken in the solid form. He does not suggest how the digested material is distributed to the other cells of the Hydra.
Lastly, we have a brief résume from Metschnikoff in the ‘Zoolog. Anzeiger,’ No. 56, May, 1880, of a series of observations on the subject of intra-cellular digestion, carried on by him in the spring at the Zoological Station at Naples.
Metschnikoff made use of carmine powder, which he observed to penetrate the endoderm cells in many Hydroid polyps and Hydromedusæ (Plumularia, Tubularia, Eucope, Oceania, Tiara, Praya, Forskalia, Hippopodius, Pelagia, Beroe among Ctenophora and Sagartia and Aiptasia among Anthozoa.) In the TrachymedUsæ Liriope, Carmarina, Cunina, Metschnikoff failed to establish the occurrence of intra-cellular digestion. It will be observed that the method employed by Metschnikoff is not altogether a conclusive one. The majority of forms studied by him, like the Hydra studied by Parker, are opaque, and, consequently, it was not possible to watch the process of ingestion during life. In Praya, however, Metschnikoff studied a transparent form, and was able to observe the throwing out of pseudopodia by the endoderm cells, and their fusion into a plasmodium. Even here, however, it seems that there is still room for doubt as to whether the pseudopodia are really active in digestion, for Metschnikoff only speaks of their penetration by carmine particles. It is exceedingly probable that when his observations appear at greater length, we shall find that they include the fact of inception of natural food materials, such as Algæ, disintegrated Entomostraca, &c. The mere penetration of minute particles like those of powdered carmine into amoeboid cells would not in itself indicate a natural process of intra-cellular digestion. Such a penetration of carmine particles into the amoeboid corpuscles of vertebrate blood is well known, and does not in that case lead to the inference of normally occurring intra-cellular digestion.
On this account I think some importance attaches to the observations which I made last summer on the intra-cellular digestion of Limnocodium, the fresh-water Medusa discovered in the lily-house of the Botanical Gardens, Regent’s Park, London. I was able, in this animal, on account of its exceeding transparency, to study the endoderm cells during life, and to establish the fact of the inception of natural food materials by those cells.
I have since made a careful study of the endoderm of various regions of this Medusa’s body in specimens preserved in osmic acid.
The series of questions which arise in connection with this phenomenon of intra-cellular digestion are so numerous and important that it is quite certain that the most complete study of the endoderm of the various regions of the digestive tract is necessary before the phenomenon can be rightly appreciated. The following considerations, amongst others, are those which naturally present themselves to an observer as indications directing his inquiries.
Preliminary Considerations
Supposing it to be established that some of the endoderm cells in Hydroid and Anthozoan polyps are capable of ingesting solid food particles, the question arises whether this is an occasional and accidental phenomenon, or whether it is a normal and definitely fixed function of such endoderm cells.
The question also occurs as to whether all the endoderm cells have this property, or whether it is limited to certain groups of these cells, whilst a distinct kind of activity (possibly similar to that of the gastric cells of other animals) is assigned to other cells in the same animal.
Further, it is of fundamental importance to ascertain what becomes of food particles ingested by amoeboid endoderm cells. Are these particles digested by these cells as food particles are by an Amœba? or are they again ejected unchanged.
Supposing the food particles to be digested—that is to say, dissolved and converted into diffusible peptones or analogous substances—what becomes of such peptones ? Are they simply retained by the endoderm cell for its own nutrition ? or are they passed by that cell away from its surface to subjacent cells, which thus are nourished by a process of diffusion ? or, again, are the products of digestion returned by the endoderm cell to the alimentary tract, and carried thence by its ramifications, as a nutrient fluid, into various regions of the body ?
Are there any special kinds of food particles which are ingested in the solid form by certain endoderm cells, whilst other food materials are dissolved and distributed by diffusion, in the same way as are albuminoids and carbo-hydrates, by the alimentary organs of Vertebrata. Is there any ground for supposing that the ingestion of fats in a particulate form by Verrebrata is a survival of the intracellular digestion now established as occurring in Cœlentera and Planarians ?
When we take into consideration the structure of Hydra it seems possible that the sole nutrition of the ectoderm cells is, by means of the products of digestion, elaborated by endoderm cells, such products passing through from the endoderm cells to ectoderm cells by osmosis. And we have definite observations of Metschnikoff (in the case of Ctenophora) upon the passage of carmine particles, away from the endoderm cells which took them up, into mesoderm cells lying beneath them, which favour the notion of such a passage. It may, however, be noted that the carmine particles do not appear in this case to have been digested—that is, chemically changed and dissolved—and hence the passage of the particles in question is a phenomenon similar, in essential respects, to the passage of fat particles unchanged through cells on the surface of the intestinal villi of Vertebrates to subjacent cells and cell spaces.
On the other hand, when we try to bring the structure of the Medusæ, with their elaborate gastro-vascular canal system, into relation with the facts of intra-cellular digestion, we find it impossible to admit that the nutrition of the organism can be carried on by the mere osmotic passage of nutrient matters from those cells which are active as intra-cellular digesters to subjacent cells. Metschnikoff has observed that in many Cœlentera the intra-cellularly digestive cells are limited in number and position, and this fact I can fully establish by my observations on Limnocodium. Hence the regions in which subjacent cells can be nourished by superjacent intra-cellularly digestive cells is exceedingly limited. The products of the digestive activity of the intra-cellularly digesting endoderm cells are in all probability, in the Medusæ, returned to the alimentary canal, and carried on by the agency of the gastro-vascular canals into the remoter parts of the organism.
Bearing in mind these considerations we may proceed to an examination of the endoderm of the gastric and gastro-vascular cavities of Limnocodium.
Intra-cellular Digestion in the Proximal Region of the Gastric tube Observed During Life
The manubrium of Limnocodium is a somewhat quadrangular tube, which depends during life below the margin of the umbrella. Its cavity, the stomach, presents a considerable difference in the structure of its lining cells, the gastric endoderm, in different regions. Where the four angles of the stomach-tube are inserted into the umbrella they are slightly produced, and give rise to the four radiating canals. The enlarged angles of the stomach are lined by peculiar cells, in which I observed an intra-cellular digestion to be proceeding during the observation of living specimens. In Plate VIII, figs. 1 and 2, two drawings of this intra-cellularly digestive endoderm, taken from living specimens, are reproduced. The cells are seen to form a widely-open meshwork, large spaces occurring between neighbouring cells, the cells being connected by ridges, which traverse the spaces. The cells themselves appear to be naked, and their protoplasm is irregularly aggregated so as to form masses with pseudopodia -like processes, and also clothing the ridges connecting cell with cell. Spherical nuclei (a), with spherical nucleoli, are placed at intervals in the protoplasm, and have a uniform appearance and size, which is characteristic of the endodermal nuclei throughout the gastric and gastro-vascular area. They measure about 
th inch in diameter, whilst the spaces in the meshwork are on an average about 
th by 
th of an inch in the smallest and largest diameter. In the neighbourhood of the nuclei are numerous dense-looking masses of an ill-defined shape, which give to the cell-substance a certain opacity (b). In fig. 2 there are also seen vacuole-like spaces or clearer portions of the cell-substance, containing very dark. minute granules.
In both figures there are seen embedded in the protoplasmic net-work green unicellular organisms.
In fig. 1 a large Euglena-like form (x) is embedded in a plasmodium formed by the confluence of cell-substance from some four or five cells. In the upper part of the figure two Protococci are seen embedded in pseudopodia-like processes of the cell network. The one to the left (y) is in a state of disintegration, that to the right (z) has not yet been altered appreciably.
In fig. 2 the letter x points to an ingested organism, which has been almost entirely broken up and its colouring matter lost; y marks a Protococcus reduced to the condition of a few coloured granules, whilst z is placed near a recently ingested Protococcus.
I did not observe the movement of the pseudopodia-like lobes of this protoplasmic network during life, nor the actual process of the entry of a solid food particle into its substance.
I may mention in this connection that the proximal region of the stomach in many specimens of Limnocodium was infested by a remarkable little free smiraming, yet tubicolous Rotifer, which carried its tube about with it as it swam.
This parasite appeared to escape altogether the embraces of the amoeboid endoderm cells, as well as to be unaffected by the digestive secretions, if any such were present.
Structure of the Gastric Endoderm of Various Regions, as seen on Treatment with Reagents
The true structure of the endoderm of the gastric tube becomes evident when specimens which have been treated with osmic acid are stained with picro-carmine and examined under the highest powers of the microscope by means of teazing and sections. The meshwork of amoeboid cells in which intra-cellular digestion takes place is seen to be confined to the four proximal angles of the gastric tube.
Endoderm, of radial canals
The endoderm suddenly changes its character at the commencement of the radial canals (see Plate IX, fig. 8 w), and in these continuations of the gastric chamber, instead of a network, we find closely-set nuclei, the cell areas not distinctly marked off from one another and the protoplasm free from granulations. These cells as seen in the living condition are ciliated.
The nuclei are precisely similar in form and size to those of the gastric tube, and take up the carmine staining in a way which is characteristic of the endoderm nuclei in general (see Plate).
Endoderm of the ring-canal
I have in my former paper on Limnocodium (this Journal, July, 1880) described and figured (Plate XXX, fig. 6) the modification of the endoderm cells on the abumbral wall of the marginal ring-canal. The cells of the adumbral wall are like those of the ring-canals. The cells of the abumbral wall are modified by the deposit of block-like masses of a dense substance within them, which usually obscure the nuclei. These cells also have a remarkably angular and irregular form. They form the representative in Limnocodium of the cartilaginous marginal ring of Trachymedusæ, and are drawn out into lobes which are continuous with the roots of the tentacles. The endoderm of the gonads (genital pouches) has a similar structure to that of the abumbral wall of the ring canal.
Endoderm of the gonads
A portion of this part of the endoderm is drawn in Plate IX, fig. 9. It quite closely resembles that of the abumbral wall of the ring-canal. The block-like deposits within the cells and the dark colour which the whole layer had assumed under the influence of osmic acid were sufficient to obscure the nuclei, which accordingly are not seen in the drawing.
Endoderm of the middle third of the gastric tube
This is represented in Plate X, figs. 1 and 2. Over a comparatively small area the cells present a uniform hexagonal pavement when viewed from their free surface (Plate X, fig. 2). The nuclei have the same size and character as in the other endoderm cells, but the cell substance is small in quantity and of a homogeneous appearance. Here and there in this and in other parts of the gastric tube, nematocysts are scattered in considerable numbers. They sometimes are embedded in the endoderm (g g) so as to present a spherical appearance, and the first explanation of their appearance here which suggests itself, is that they have been developed in endoderm cells. But the fact that they are scattered very irregularly and occur in all regions of the gastric tube sporadically is against this view. Further the absence of any cells of the endoderm in which stages of the development of such nematocysts can be made out is also against the view that they are developed here. Lastly, the facts that they are precisely similar in appearance and size to the nematocysts of the tentacles, and that actual bits of ectoderm cells containing three or four nematocysts side by side may be observed occasionally in the gastric tube, are in favour of the view that the nematocysts occurring in the gastric endoderm have been swallowed by the Medusa with its prey, and have become embedded in the soft endoderm fortuitously.
This explanation has been offered by Mr. Marcus Hartogg (see this Journal, 1880) of the similar occurrence of nematocysts in the endoderm cells of Hydra; and for the present case, as well as that of Hydra, it seems to me to be satisfactory, though it must be remembered that there is no great improbability connected with the development of nematocysts by endoderm cells unless the mesenterial filaments of the Anthozoa can be shown to have an ectodermal origin.
Above and below the limited region of homogeneous hexagonal cells the endoderm of the middle third of the gastric tube exhibits two distinct concomitant modifications (Plate X, fig-I):
Some of the cells are enlarged and highly granular (b), in fact have become secretion cells or unicellular glands.
The cells are no longer continuous, but here and there the cell-pavement is deficient, actual gaps of greater or less size (f) making their appearance between neighbouring cells..
Endoderm of the oral third of the gastric tube
The endoderm of the oral region presents a condition which may be considered as a development of that last described. In Plate IX, fig. 3, a piece is represented. All the cells are here either fully developed as secretion cells (b), large clear bodies about the 
th inch in diameter, or are on their way to this condition (h). The nuclei have the characteristic form and size (a). The intercellular spaces (f) are very small and few, whilst surrounding the enlarged secretion cells and enclosing the yet young secretion cells is a sort of laminated matrix (d). This matrix is to be regarded as an intercellular substance of a horny or gelatiginous character. It forms a complete framework to the whole series of cells, enveloping each of the more fully-grown secretion cells in a distinct capsule, which is broken through on the free surface of the endoderm by circular apertures (Plate IX, fig. 4) corresponding each to a ripe secretion cell.
The nuclei of the ripe secretion cells are less defined than those of the younger cells, and I am inclined to think that they undergo atrophy, and that the whole secretion cell, when its chemical metamorphosis is complete, is passed into the gastric cavity. I am also led to believe that this takes place periodically by the following observation.
Whilst in some specimens of Limnocodium studied by me the oral gastric endoderm presented uniformly the appearance represented in Plate IX, fig. 3, yet in another batch of specimens it had uniformly a very different appearance, which is drawn in Plate IX, fig. 6. In this case all the sites which in the former example were occupied by large-sized secretion cells are empty (f). The framework (d) remains, and projecting into the empty spaces, as though destined in their turn to occupy them, are small secretion cells (b).
I can only interpret these appearances on the supposition that the large cells are shed when ripe, and that the next generation grow out into the spaces left, whilst a third generation is developed from the scattered cells, with at present little protoplasm, and merely indicated by the nuclei (a a). And, further, it seems that the ripening and shedding of the secretion cells must take place in the whole of the oral gastric endoderm simultaneously.
It is possible that a periodicity of this kind may be inherent in the growth and development of these cells. It is also exceedingly likely that the simultaneous clearing off of all the ripe secretion cells is due to some special act of the Medusa. It is likely that the act of feeding, of seizing prey, such as Entomostraca (on which the Medusa was frequently seen to feed), would be the determining cause of the clearing out of the secretion cells.
This hypothesis is borne out by some further facts, to be related below.
Whether it be accepted or not, it is clear that we have a copious secretion produced by the oral-gastric endoderm, and it is in the highest degree probable that this secretion has the action of a ferment or of a solvent upon the larger food masses taken into its gastric tube by Limnocodium.
A modification of the endoderm, not unlike this of the oralgastric region of Limnocodium, is described by Claus in Charybdaa marsupialis, that most interesting of all Medusæ. In his admirable memoir on Charybdæa (‘Arbeiten des Zoolog. Instituts zu Wien,’ 1878) Claus gives, in his plate iv, figs. 36 and 37, drawings of endoderm from the oral portion of the gastric tube, closely resembling that figured by me in Plate IX, fig. 3. Claus distinguishes two kinds of gland cells, corresponding to what I believe to be young and old stages of one kind of gland cell. A difference exists in the fact that in Charybdæa ciliated cells are interspersed among the gland cells, whilst such do not appear to be present in the same region in Limnocodium.
Endoderm of the proximal third of the gastric tube.—As we pass upwards towards the umbrella, along the walls of the gastric tube, the endoderm cells gradually open out, leaving intercellular spaces, and where the tube expands slightly in the horizontal plane the characters exhibited in Plate IX, figs. 1 and 2, are assumed. This is the region which has already been described above in the living condition, and in which intra-eellular digestion takes place.
A comparison of figs. 1 and 2, Plate IX, with figs. 3 and 6 of the same plate, shows that we have in this region the same elements of form to deal with as in the oral region, but somewhat differently characterised. There are large inter-cellular spaces (f), which are marked off by a somewhat fibrillated or laminated framework (d); spherical nuclei, which take the carmine staining, are scattered irregularly, and have surrounding them a protoplasmic cell substance, which is very deficient in some parts, and is aggregated in other parts; it appears to be continuous throughout, and is not marked off into separate cells corresponding to the individual nuclei. Two nuclei are often closely approximated, indicating recent division, but I have not met with any in process of division.
Corresponding to the secretion cells of the oral-gastric endoderm are circular or oblong groups of oval bodies of a refringent substance (b), which appear to correspond to the groups of large granules seen in the living specimens. As now seen (after the action of reagents), these groups appear to be formed by oval droplets of a homogeneous transparent substance, which stain of a pale-pink colour with picro-carmine, and are strongly emarginated by the difference of refractive index between their substance and that of the material in which they are deposited. Whilst representing, in position and size, the secretion cells of the oral-gastric endoderm, these bodies have a different structure from those cells, and the substance which stains pink is unlike anything present in that region.
Large vacuole-like spaces also occur (e e), in which a few dark granules and irregular particles may be observed, whilst the substance filling the vacuole is transparent, and stains pink with picro-carmine. It also appears to have been precipitated, as a homogeneous or excessively finely granular solid by the action of the reagents.
The substance filling the vacuoles (e) is apparently identical with the substance filling the numerous oval spaces of the bodies (b b). At the same time there can be little doubt, from the comparison of the prepared specimens with the living, that the vacuoles are food vacuoles, viz. spaces into which solid food materials have been taken and digested. Accordingly the material which they contain is an albuminous substance resulting from the digestion of those food particles.
From these considerations it seems not improbable that the pink substance of the bodies (b) is also an albuminous substance resulting from digestive activity.
I submit as suggestions for further examination, when the histology and physiology of the endodorm is attempted in other Medusæ, that these bodies (ó) are either points at which numerous small food particles have been incepted and digested by the protoplasm, or, what is more probable, that they are portions of the protoplasm of this remarkable meshwork which are especially active in “working up “the products of intra-cellular digestion, and that they periodically discharge the albuminous product of digestion and elaboration into the gastric chamber, whence it passes into the radial canals and marginal canal to nourish the outstanding parts of the organism.
That albuminous substances in a digested state must pass into these canals, either in this way or as the result of the digestion of a portion of the food by juices secreted into the gastric cavity, appears obvious when the limited number and area of the intra-cellularly digestive cells is considered.
Projecting into the spaces (f) of the mesh work are pseudopodia-like processes (c in figs. 1 and 2, Plate IX); these are not only given off from the larger masses of cell-substances, but appear to spread along the fibro-laminar trabeculæ (d) of the meshwork, and whilst clothing the trabeculæ, and often projecting from them into the inter-cellular spaces, also keep the protoplasm of neighbouring masses in continuity.
Just as in the oral-gastric endoderm, two very different conditions of nourishment and activity were observed, so here in the endoderm of the proximal end of the gastric tube—which I will call the ingestive endoderm—there were two very different conditions which came under my observation. The two conditions of the ingestive endoderm were definitely related to the two conditions of the oral endoderm. When the oral endoderm presented the condition of abundant large secretion cells filling up the inter-cellular spaces (Plate IX, fig. 3), then the ingestive endoderm had the appearance just described (Plate IX, figs. 1 and 2). It was active in throwing out pseudopodia into the large inter-cellular spaces, and was feeding upon the small particles (such as Protococci and Euglenæ) which chance threw in its way. In fact, whilst the oral endoderm was full and unshed, the ingestive endoderm at the other end of the gastric tube was half-starved, with great inter-cellular spaces and eager pseusopodial processes, making the best of bad times, and taking up materials previously unprepared.
In those specimens, however, in which the oral endoderm had shed its secretion cells, and in which I have supposed that an act of swallowing some large prey had recently taken place—in these the ingestive endoderm of the proximal end was totally changed in appearance. It was gorged with finely granular matter; its inter-cellular spaces had almost entirely disappeared in consequence of the swelling out of the protoplasm, now remarkable for its granular structure.
The appearance is represented in Plate XI, fig. 5. The masses of oval metamorphic products (b b) are still present, but the spaces are reduced to a few small chinks (f). The trabeculæ of the framework are no longer visible, owing to the swelling of the protoplasm and its granular opaque character; they are concealed by the contiguous edges of the enlarged masses of protoplasm.
I conceive this change to be due to the absorption by the ingestive cells of a very abundant supply of albuminous matters obtained by the digestion in the cavity of the gastric tube of a Daphnia, Cyclops, or some such form. The raw products of gastric digestion—partly dissolved partly in the form of fine particles—would, it may be assumed, be taken up by the amoeboid ingestive cells, just as are the rarer living food-particles in times of dearth when so copious a feast as that afforded by a Daphnia is not forthcoming.
As to the return of the ingestive endoderm to its meshwork state, with pseudopodia ready for the inception of large foodbodies, I have no observations to offer, and I will not speculate further upon the possible activity of the ingestive endoderm in elaborating the food matters taken in by it.
It is a matter for regret that the fresh-water Medusa, died down in the lily-house tank a few weeks after its discovery, so that I have not been able to follow up experimentally some of the suggestions which the study of the endoderm has afforded me. It would be an easy matter with Limnocodium and, indeed, with other small Medusæ, to determine experimentally the condition of the endoderm cells of different regions of the gastric tube before, during, and after the introduction into that tube of an Entomostracous Crustacean.
The observations and interpretations which I have put forward in the preceding pages cannot be regarded as more than an early contribution to the subject of intra-cellular digestion and the comparative physiology of digestion in general, which I do not doubt is about to be investigated with new vigour and interest, in consequence of Metschnikoff’s researches.
Summary
The cells of the endoderm of the gastric tube and gastrovascular canals differ very considerably in form and in the chemical metamorphosis of its substance in different regions.
The nuclei are alike in all as to size and form, excepting in the cells of the abumbral wall of the marginal canal and the similar cells of the endoderm of the genital pouches.
These latter are angular, close-set cells, with dense blocklike deposits in their protoplasm concealing the nucleus.
The cells of the radial canals are close set and ciliate with sparse, hyaline protoplasm.
The endoderm of the gastric tube is divisible into three regions: a, the oral, b, the mid-gastric and c, the ingestivo or proximal.
Only the cells of the proximal region exhibit intra-cellular digestion.
The cells of the oral region produce a secretion by their development as secretion cells (goblet cells of Claus).
The cells of the mid-region are inactive.
The cells of the proximal region appear, under certain circumstances, as an open meshwork giving off amœboid processes, by means of which they take in solid food particles.
Under the same circumstances the secretion-cells of the oral region are richly developed and in place.
Under other circumstances the cells of the oral region appear to have been, to a large extent; shed, leaving inter-cellular spaces.
When this is the case, the secretion-cells of the proximal are swollen and granular, and the inter-cellular spaces of the mesh work obliterated.
It is inferred that the latter circumstances are the result of the taking into the gastric tube of relatively large prey; whilst the former condition is one of comparative fasting, in which such small food bodies as may be ingested by the endoderm of the proximal region are proportionately valuable to the organism.
Addendum on the Endoderm of the Tentacles
In Plate X, fig. 3, a surface view is given of one of the smallest sized tentacles, for the purpose of showing the mode in which the thread-cells are clustered in groups upon its surface.
These groups appear to have a spiral arrangement, more or less definitely expressed. In fig. 6 two thread-cells are represented with ejected filament, showing the series of six small barbs at its base. In fig. 4 an optical median longitudinal section of the tentacle is drawn, in order to show definitely the character and arrangement of the endoderm cells. The specimen from which the drawing was taken had been treated with osmic acid and picro-carmine. An actual transverse section of a similar tentacle is shown in fig. 5. The endoderm cells consist of a dense, highly-refringent substance, which is somewhat wrinkled by the action of the reagent. The nuclei are a little smaller than those of the gastric endoderm. In some cases a small amount of granular cell substance may be seen radiating from the nucleus, but the whole cell body otherwise has been metamorphosed into a homogeneous cartilaginoid substance. There is no continuous lumen, although the cells are disposed in a single series around the axis of the tentacle, and leave, on shrinking, a small space where their adaxial surfaces should come into contact. This potential lumen appears not to be continuous, even in the specimens treated by reagents, and in living specimens it has no existence.
A structureless lamella (Stutz-lamella) (c) adheres closely to the endoderm cells. Subjacent to the ectoderm cells are the very fine transversely-striped muscular processes (d), which are developed on their inner faces, not only here but in the case of the subumbrellar ectoderm and of the ectoderm of the adumbrellar surface of the velum.
