Semper’s organ has been studied and compared in two genera, Limax and Helix. In both cases the organ, which lies on each side of, and beneath, the buccal mass, is composed of glandular elements surrounding ganglia; the latter originate from two nerves arising from each side of the cerebral ganglia. In both, the organ produces the external swellings that form the mouth lobes on each side of the head. The chief difference between the two is that Semper’s organ is much larger in Limax. In Helix it is reduced to strands of secretory and nervous tissue, dispersed among abundant muscle-fibres.

In H. aspersa the glandular area surrounding the ganglionic part of the organ is composed of three cell-types. The cytoplasm of the chief cellular component, ‘Semper’s’ cells, contains a number of lipochondria, some of which possess the cortex of concentric lamellae typical of phospholipid globules. The cytoplasm of the second cell-type, or ‘mucus’ cell, is filled with electron-lucent globules between which are scattered electron-dense bodies that contain some polysaccharide and lipid. The third cell-type, the reticulate cell, is rather similar to the mucus cell, except that its cytoplasm contains a reticulum formed by strands which lie within, or applied to, the electronlucent globules; these cells contain acid mucopolysaccharides. The structure of the mucus and reticulate cells bears certain resemblances to secretory cells in the sheath of the optic tentacles in the same species.

The function of Semper’s organ is discussed. Certain evidence suggests that some of its secretory cells may have an endocrine activity, and the significance of lipochondria in such actively secreting invertebrate cells is considered.

IN 1856 Semper described glandular lobes in the cephalic region of certain pulmonates, since then referred to as ‘Semper’s organ’. The component parts of these lobes, lying beside the buccal mass in close association with the tentacles, have subsequently been reappraised and assigned to different categories, nervous and glandular (Eckardt, 1914). Semper’s organ varies in size in different pulmonates, being largest in the slug Limax (Semper, 1856)….

Investigators have studied the arrangement and anatomical details of Semper’s organ in different genera, and many have put forward suggestions as to its possible function (Flemming, 1869; Leydig, 1876; Simroth, 1876, 1891; Sochaczewer, 1881; Sarasin, 1883; Hanitsch, 1888; Babor, 1895; Tauber, 1900; Beutler, 1901; Keller, 1902; Hâckel, 1911; Meisenheimer, 1912; Eckardt, 1914; Hoffman, 1925; Bronn, 1928; Rotarides, 1929; Baecker, 1932). In a preliminary examination I found that both in Semper’s organ and in the tentacles of the Stylommatophora, secretory cells are arranged around ganglia; further, certain cells in Semper’s organ resemble some of the secretory cells in the optic tentacles (Lane, 1962). This similarity to cells in the tentacular sheaths has also been mentioned by several earlier workers (Eckardt, 1914; Bronn, 1928; Baecker, 1932). Since the cells in the optic tentacles have been found to produce a hormonal effect on the ovotestis (Pelluet and Lane, 1961), it seemed worth while to examine Semper’s organ in greater detail, to determine to what extent the structure of its component cells paralleled that of the tentacular secretory cells and whether its cells were also responsible for the production of secretions which might be hormonally active.

No previous histochemical or detailed morphological investigation appears to have been made of the cells that compose the glandular portion of Semper’s organ.

The animal chiefly used in this study was the common garden snail, H. aspersa Müller, which was employed throughout all the histochemical and electron microscopical investigations. The slug Limax flavus Linnaeus was also used to make a comparison between the structure of Semper’s organ in the two genera. Serial paraflin sections were made of the entire head region of H. aspersa, which had been fixed with the tentacles extended; these sections were stained with haematal 8/Biebrich scarlet (Baker, 1962) in order to ascertain the organization and localization of Semper’s organ. For the histochemical studies, either Semper’s organ or the whole cephalic area was removed and fixed in Helly’s solution or formaldehyde-calcium followed by post-chroming (Baker, 1944); the tissue was then embedded in paraffin or gelatine respectively. The histochemical tests performed, with the details of preparation, are summarized in the appendix (p. 342).

Tissues for the electron microscopical study were fixed in Palade’s buffered osmium solution (1952), dehydrated in an ascending series of alcohols, treated with propylene oxide, and embedded in araldite according to Luft’s specifications (1961). Sections showing silver or gold interference colours were cut on the Huxley ultramicrotome and mounted on uncoated or formvar-filmed grids. In some cases the sections were stained with potassium permanganate for 10 min before inspection, to increase the contrast (Lawn, 1960). The specimens were examined in an Akashi electron microscope (model TRS 5OEI).

Araldite sections were also cut at 0-25 p. on the ultramicrotome, placed on pieces of glass coverslips, and dried. After being stained for 1 min in 1% toluidine blue borax solution and differentiated with 50% ethanol, they were mounted on slides under liquid paraffin (Meek, 1963). These preparations were examined by light microscopy in order to establish the presence of the cells comprising Semper’s organ, and to determine their localization in the section, before the thinner serial sections were examined under the electron microscope. This precaution is especially necessary in H. aspersa, because the cells of the gland are dispersed amongst muscle-fibres and often do not appear in sections.

The description of Semper’s organ by Eckardt (1914) has been accepted by most subsequent workers as definitive. In a study of Limax and Agriolimax he concluded that Semper’s organ consisted of diverse component parts: ‘lip ganglia’ and ‘mouth lobe ganglia’, surrounded by packets and layers of unicellular glands which form the ‘mouth lobe glands’. Eckardt considered that the mouth lobe ganglia were innervated by thick nerves that arose from each side of the cerebral ganglia and ran to the inferior tentacles. He also found that certain superficial cells in the glandular packets discharged their products externally, and he considered that they thereby functionally supported the lateral belt of the foot glands; he was, however, not able to discover for the other, non-superficial, cells, any ducts to the exterior or to the digestive tract.

My findings are on the whole in agreement with those of Eckardt. For example; I find that some, but not all, of the cells in the glandular portion of Semper’s organ have ducts to the external surface, and none have ducts to the digestive tract. In Limax, however, in the animals I have examined, I have found 2 nerves arising side by side from each cerebral ganglion, of which one runs to the inferior tentacle, and the other to the lobes of Semper’s organ (fig. 1, A), albeit there is a branch from the latter nerve across to the inferior tentacle.

FIG. 1.

Diagrams of dissections, viewed dorsally, made on one side of the cephalic region in A, Limax flavus and B, H. aspersa, illustrating the arrangement of Semper’s organ in relation to the inferior and optic tentacles, brain, and buccal mass. The cephalic area of the snail in B contains the same organs as those labelled for the slug in A, with the exceptions noted. None of the other nerves arising from the cerebral ganglion are included in this diagram.

FIG. 1.

Diagrams of dissections, viewed dorsally, made on one side of the cephalic region in A, Limax flavus and B, H. aspersa, illustrating the arrangement of Semper’s organ in relation to the inferior and optic tentacles, brain, and buccal mass. The cephalic area of the snail in B contains the same organs as those labelled for the slug in A, with the exceptions noted. None of the other nerves arising from the cerebral ganglion are included in this diagram.

I have also found 2 other smaller nerves arising from each side of the cerebral ganglia just beside those innervating the tentacles and the lobes of Semper’s organ; each of these runs ventrally under the buccal mass to terminate in a small labial ganglion (fig. 1, A), around which lie cords of secretory tissue. These no doubt correspond to Eckardt’s Lippenganglien, and the 2 main lobes on each side of the buccal mass (fig. 1, A) may be identified with his mouth lobe ganglia and glands. The mouth lobe ganglia and glands would appear to produce the external swellings of the mouth lobes, one on each side of the head between the inferior tentacles and the foot (similar to the arrangement in fig. 2).

FIG. 2.

Diagram to illustrate an anterior external view of the cephalic region of H. aspersa.

FIG. 2.

Diagram to illustrate an anterior external view of the cephalic region of H. aspersa.

In Limax, Semper’s organ can easily be seen and dissected out as a white or white-grey mass. In Helix it is not so immediately evident, for it is much smaller. It lies in the same site, but the organ is restricted to a small area at the end of a number of muscular strands, situated just beside the point where the sheath of the inferior tentacles arises (figs, 1, B ; 4, A). The gland, therefore, is not readily dissected out, and the cells can most easily be located in serial sections of the whole cephalic area. Externally, however, the mouth lobes, within which the gland cells and ganglia are situated, are quite extensive swellings (fig. 2).

The structure of Semper’s organ in Helix differs from that in Limax, then, as shown in fig. 1. Not only is the organ smaller, but the muscular strands between which its secretory cells lie continue in a posterior direction to form a muscle band that joins up with the retractor muscle of each optic tentacle (fig. i, B). This does not occur in Limax. Further, the innervation of the organ differs, in that in Helix only one nerve arises from the cerebral ganglion to supply both Semper’s organ and the inferior tentacle (fig. 1, B).

In addition to the numerous nerve-fibres from the ganglia running among the secretory cells of Semper’s organ, there is an extensive vascular supply. In one section from H. aspersa, the duct of a cell from the organ appeared to run between the epithelial cells directly into a blood-vessel (fig. 4, c).

At the ultrastructural level, the surface of the mouth lobe area in H. aspersa is covered with a cuticle composed of microvilli, similar to the cuticle on the optic tentacles and the dorsal body surface (Lane, 19636). In the mouth lobe and cephalic area generally, unicellular calcium glands are present beneath the surface epithelium, like those in the optic tentacles (Lane, 1963e). Ultrastructurally these appear to correspond to cells composed of vast quantities of spheroids and ellipsoids of varying size, having a concentric layered structure (probably the form taken by the calcium salt of which they are composed).

In H. aspersa, three different cell-types are found in the glandular area of Semper’s organ; these cells lie around the ganglia of the organ and the nerve-fibres arising from them. It is difficult to delimit the glandular portion proper, as many of the cells are drawn out into long processes and lie between the nerve and muscle-fibres in cords or packets (fig. 4, A). The cells which I propose to describe form what could be considered the mouth lobe gland area, in that they lie around the ganglionic regions and nerve-fibres on each side of the buccal mass. Other secretory cells, lying beneath the mouth cavity and forming the glands of the lip ganglia, appear to be similar to the cell- types to be described, except that the cells corresponding to the third cell- type occur only infrequently.

The chief cell-type, which comprises the bulk of the glandular area, will be referred to as ‘Semper’s’ cells. They are ellipsoidal and measure up to 30 p. in length and 15 p in width. Their cytoplasm contains a great many spheroidal or ellipsoidal granules from 1 to 2 p in diameter (figs. 3, A; 4, B). In some cases vacuoles, 1 to 2 p in diameter, are also present. The nuclei of Semper’s cells are spheroidal, up to 9 p in diameter ; often, however, they are indistinguishable, completely obscured by the quantities of granules in the cell. The duct that entered a blood-vessel directly, as described earlier (fig. 4, c), contained granules similar to those in Semper’s cells ; a number of these cells lay around the blood-vessel and presumably the duct arose from one of them. A few of Semper’s cells have ducts running to the external surface, but most of them do not appear to do so.

FIG. 3.

Diagram to illustrate the differences between the three secretory cell-types that make up Semper’s organ in H. aspersa, as they appear after staining with chrome-haematoxylin-phloxine. A, Semper’s cell; B, mucus cell; c, reticulate cell.

FIG. 3.

Diagram to illustrate the differences between the three secretory cell-types that make up Semper’s organ in H. aspersa, as they appear after staining with chrome-haematoxylin-phloxine. A, Semper’s cell; B, mucus cell; c, reticulate cell.

FIG. 4.

(plate). All the micrographs shown in this figure are from sections of Semper’s organ in H. aspersa.

a section through Semper’s organ, showing how its secretory cells (sc), lying among muscle-fibres (mf), are found close by the sheath of the inferior tentacle (inf). Note the nerve (ne) from the cerebral ganglion running into the organ, as well as blood-vessels (bv). Helly/haematal 8/Biebrich scarlet.

a Semper’s cell (s), the chief cellular constituent of Semper’s organ. Note the spheroidal and ellipsoidal granules (g) which almost fill the cytoplasm. Brasil/chrome-haematoxylinphloxine.

c, a cellular process (p), apparently from a Semper’s cell, running directly into the lumen (I) of a blood-vessel (br). Helly/haematal 8/Biebrich scarlet.

D, mucus cells (me) located alongside Semper’s cells (s). n, nucleus ; p, process ; gl, globules ; gr, granules lying between the globules. Brasil/chrome-haematoxylin-phloxine.

E, a reticulate cell (re) situated on the periphery of Semper’s organ, Note the globules (gl) with strands (st) lying upon them or inside them, gr, smaller granules scattered among the globules; p, cellular process. Helly/haematal 8/Biebrich scarlet.

FIG. 4.

(plate). All the micrographs shown in this figure are from sections of Semper’s organ in H. aspersa.

a section through Semper’s organ, showing how its secretory cells (sc), lying among muscle-fibres (mf), are found close by the sheath of the inferior tentacle (inf). Note the nerve (ne) from the cerebral ganglion running into the organ, as well as blood-vessels (bv). Helly/haematal 8/Biebrich scarlet.

a Semper’s cell (s), the chief cellular constituent of Semper’s organ. Note the spheroidal and ellipsoidal granules (g) which almost fill the cytoplasm. Brasil/chrome-haematoxylinphloxine.

c, a cellular process (p), apparently from a Semper’s cell, running directly into the lumen (I) of a blood-vessel (br). Helly/haematal 8/Biebrich scarlet.

D, mucus cells (me) located alongside Semper’s cells (s). n, nucleus ; p, process ; gl, globules ; gr, granules lying between the globules. Brasil/chrome-haematoxylin-phloxine.

E, a reticulate cell (re) situated on the periphery of Semper’s organ, Note the globules (gl) with strands (st) lying upon them or inside them, gr, smaller granules scattered among the globules; p, cellular process. Helly/haematal 8/Biebrich scarlet.

The second cell-type is frequently found lying between Semper’s cells, but, in serial sections, it can also be seen in abundance in the area of the foot gland. I shall refer to these cells provisionally as ‘mucus’ cells, since their structure is rather similar to that of some vertebrate mucus cells. They are about 33 p in length and 16 p in width, with a spheroidal nucleus of about 8 p in diameter. These mucus cells contain large quantities of spheroidal or ellipsoidal globules, 1 to 2 p in diameter, which are refractile and, in contrast to the inclusions in Semper’s cells, take up few dyes (figs. 3, B; 4, D). In some preparations a number of smaller granules can be seen lying between these globules.

The cells of the third type, in or associated with the glandular area of Semper’s organ, I shall term reticulate cells, because they contain a reticular material that can only be with difficulty resolved into globular inclusions (from less than 1 to 3 or more p. in diameter) and strands that either lie in, or are superficially applied to, the globules (figs. 3, c; 4, E). Smaller granular bodies lie between the globules and strands, but they cannot always be resolved as separate from the other two components. The reticulate cells themselves tend to be ellipsoidal and measure approximately 40 /x in length. They have spheroidal nuclei that are usually obscured by the cytoplasmic inclusions. These cells often lie at the periphery of the area in which the Semper’s and mucus cells are found. In addition, they are most extensively located in the region near the foot and in some cases have long ducts that run to the surface of the sole, opening between the cilia. These reticulate cells, therefore, may not form part of Semper’s organ proper, but are found sufficiently often between and near the other two cell-types to warrant inclusion in a description of the glandular area.

Histochemistry and fine structure of the secretory inclusions

The results of the histochemical tests applied to the cells which compose the glandular area of Semper’s organ are to be found in the appendix (p. 342).

The granules in the cytoplasm of Semper’s cells respond positively to tests for carbohydrate and for lipids in general, and also, in most cases, to a test for phospholipids; this suggests that they may be partially composed of cerebroside. These secretory granules, or another cytoplasmic component with a similar distribution, react positively to tests for ‘neurosecretory’ substances. Ultrastructurally, the granules appear to correspond to spheroids of varying electron-density and diameter (fig. 5, A, C) that contain electrondense granules and lamellae which are in some cases arranged concentrically in the cortex (fig. 5, B). Although the granules are fairly numerous within each cell, there is a good deal of cytoplasm between them which contains membranes, mitochondria, vesicles, and smaller granules (fig. 5, A, C).

FIG. 5.

(plate). All the electron micrographs shown in this figure are from Semper’s organ of H. aspersa; the tissues have been fixed in Palade’s buffered osmium and embedded in araldite.

A, a cell from the region of Semper’s organ that appears to be a Semper’s cell. Note the muscle-fibres (mf) which lie adjacent to it and the lipochondria (Ip) within its cytoplasm. gr, smaller granules; v, vesicles; m, mitochondria; mb, membranes; n, nucleus of a muscle-cel).

B, a lamellated granule, or phospholipid globule (pip), from a Semper’s cell, whose lamellae have been somewhat disrupted from their concentric arrangement. Ip, lipochondrion ; gr. granule.

C, lipochondria (Ip), showing different degrees of electron-density, in the cytoplasm of a Semper’s cell, tr, triglyceride droplets; n, nucleus; v, vesicles.

D, an electron-dense body (edb) from the cytoplasm of a mucus cell, gl, globule.

E, an area of cytoplasm from a mucus cell in Semper’s organ, containing electron-lucent globules (gl). cy, ground cytoplasm.

F, a reticulate cell from Semper’s organ containing electron-lucent globules (gl) in its cytoplasm (cy). Note the electron-dense granules and strands (st) lying in the globules.

FIG. 5.

(plate). All the electron micrographs shown in this figure are from Semper’s organ of H. aspersa; the tissues have been fixed in Palade’s buffered osmium and embedded in araldite.

A, a cell from the region of Semper’s organ that appears to be a Semper’s cell. Note the muscle-fibres (mf) which lie adjacent to it and the lipochondria (Ip) within its cytoplasm. gr, smaller granules; v, vesicles; m, mitochondria; mb, membranes; n, nucleus of a muscle-cel).

B, a lamellated granule, or phospholipid globule (pip), from a Semper’s cell, whose lamellae have been somewhat disrupted from their concentric arrangement. Ip, lipochondrion ; gr. granule.

C, lipochondria (Ip), showing different degrees of electron-density, in the cytoplasm of a Semper’s cell, tr, triglyceride droplets; n, nucleus; v, vesicles.

D, an electron-dense body (edb) from the cytoplasm of a mucus cell, gl, globule.

E, an area of cytoplasm from a mucus cell in Semper’s organ, containing electron-lucent globules (gl). cy, ground cytoplasm.

F, a reticulate cell from Semper’s organ containing electron-lucent globules (gl) in its cytoplasm (cy). Note the electron-dense granules and strands (st) lying in the globules.

The globules of the mucus cells are very non-reactive histochemically, but may contain slight amounts of carbohydrate. Under the electron microscope, they are electron-lucent, and are either empty or contain only a faint granular substance; very little cytoplasm lies between them (fig. 5, E). Occasionally, amorphous, electron-dense bodies are found dispersed between the globules (fig. 5, D); these may represent the smaller granules seen scattered among the globules under the light microscope. The smaller granules contain, in addition to some lipid, rather more polysaccharide than the globules, and they take up the ‘neurosecretory’ stains. However, it is difficult to determine their true nature, for they are not visible by light microscopy in all preparations, and their size and shape vary a great deal in electron micrographs.

In the reticulate cells, sulphated mucopolysaccharides are present, and both the globules and the strands contain acid mucopolysaccharides; some lipid is present in the strands and scattered granular bodies. The fine structure of these cells seems to differ only slightly from that of the mucus cells, though the proximity of the two cell-types makes it impossible to distinguish them with certainty under the electron microscope. However, one cell-type contains, in addition to electron-lucent globules and scattered amorphous electron-dense bodies, a number of granular lumps and strands of high electron density that lie within or upon the electron-lucent globules (fig. 5, F). These strands, in my opinion, correspond to those seen in the reticulate cells under the light microscope.

Over the years there has been confusion in the terminology of Semper’s organ. The external lobes beneath which the organ lies have been considered to represent a third pair of Fiihler (tentacles) (Leydig, 1876). They have been termed Mundlappen by some investigators (among them Semper, 1856; Eckardt, 1914; Hoffman, 1925) and Lippen by others (Simroth, 1876). The lobes of the organ have often been confused with the papillae of skin around the entrance to the mouth. However, to avoid the possibility of confusing the other tentacles or the lips round the entrance to the mouth for Semper’s organ, it seems best to use the term ‘mouth lobe’ for the external swellings formed by it. As described earlier, Eckardt’s (1914) division of the organ into mouth, lobe ganglia, mouth lobe glands, and labial ganglia in Limax seems to be established as accurate (Bronn, 1928; Baecker, 1932), though there are variations in different genera, as for example in Helix, which I have already described, in Paralimax (Tauber, 1900), or in Amalia (Bronn, 1928).

There has also been a controversy as to the actual nature and function of the cells comprising the glandular portions of Semper’s organ. Semper himself considered the secretory cells to form a Geruchsorgan. Others have variously described it, like Semper, as a gustatory sense organ, a Geschmacksorgan (Simroth, 1876), or as a mixed sensory and glandular organ (Hanitsch, 1888), as a purely glandular organ (Sochaczewer, 1881), as a slime gland pouring secretions on to the surface of the head (Rotarides, 1929), as a pharyngeal salivary gland opening into the digestive tract (Babor, 1895; Meisenheimer, 1912), or as a connective tissue element (Tauber, 1900; Bronn, 1928). Eckardt (1914) considered that the glandular cells served during gliding as padding and protection for the ganglia.

Although the ganglionic component of Semper’s organ certainly suggests a sensory function, I have no definite evidence bearing on this. Some of the superficial cells of the glandular component have an exocrine secretory function, particularly the reticulate cells that lie on the side of the organ near the foot. It is improbable that the hypothesis of Tauber (1900) and Bronn (1928) is correct, for not only do some of the gland cells have obvious ducts, but they do not lie in a matrix that is demonstrably composed of mucopoly- saccharides, like the ground substance of connective tissue. In addition, like other investigators (Semper, 1856; Eckardt, 1914; Bronn, 1928), I have not found any excretory ducts from Semper’s organ entering the digestive tract, so that it cannot be a salivary gland. Since there is an extensive blood supply, it is possible that those cells in Semper’s organ that do not have external ducts empty their secretory products into the blood-stream, perhaps directly. If this is so, the cells form an endocrine gland. It is noteworthy that Quattrini (1956; 1957) has found that the amounts of DNA in the nuclei of the cells of Semper’s organ are variable in a cyclic manner that is connected with a cytoplasmic secretory cycle. Such results are also suggestive of endocrine activity.

Certain cells in Semper’s organ bear similarities to the cells of the foot gland (Eckardt, 1914), to which area sensory (Hanitsch, 1888) and perhaps olfactory (Sochaczewer, 1881) functions have also been ascribed. Campion (1961) described cells in the pedal gland of H. aspersa which had, at least in their ducts, a ‘reticular’ structure; these she found similar to those termed the ‘sole-glands’ by other investigators. The cells, as she characterizes them, are very like the reticulate cells of Semper’s organ, which, as I have already suggested, form at least part of the foot gland. Another indication of the identity of the reticular cells in the foot with those in Semper’s organ is the difficulty Bronn found (1928) in deciding whether the ‘upper lip gland’ described by Hackel in Chitina corresponds to a mouth lobe gland or to a foot gland.

The cytological similarities between the secretory cells in Semper’s organ and the Spindelzellen in the sheath of the optic tentacles (termed the ‘lateral’ cells by myself, 1962) have been noted previously by Eckardt (1914) and Baecker (1932). Both groups of cells are characterized by a lack of excretory ducts (Bronn, 1928). The mucus and reticulate cells in Semper’s organ bear strong resemblances to the lateral oval and lateral processed cells of the optic tentacles respectively, both by light and electron microscopical methods (Lane, 1962; 1963c), although there are certain differences. Ultrastructural similarities are to be found between both the lateral cells of the tentacles and the mucus and reticulate cells on one hand, and certain vertebrate mucus cells on the other (Lane, 1963c; Florey, 1960; Shearman and Muir, 1960; Palay, 1958; Rhodin and Dalhamn, 1956). In particular, electron-dense bodies of similar structure (as in fig. 5, D) are found in all the afore-mentioned cell-types. In the mucus and reticulate cells, and the tentacular cells, these bodies contain lipid; in the latter, they are also the sites of acid phosphatase activity, which suggests that they may correspond to lysosomes (Lane, 1963d).

The granular inclusions in Semper’s cells contain carbohydrate and lipid, with a partial phospholipid component. In correspondence, their fine structure is similar to that of lipochondria such as are found, for example, in the neurones of Helix (Chou and Meek, 1958; Dalton, 1960; Meek and Lane, 1964), and of Locusta (Ashhurst and Chapman, 1962). In some cases the granules resemble non-crenated triglyceride droplets or mixed lipid globules (fig. 5, c); in others they contain the concentric cortical lamellae typical of phospholipid globules (Chou and Meek, 1958; Mercer, 1962; Meek and Lane, 1964). In spite of the fact that at the light microscopical level the ‘neurosecretory’ stains are taken up by Semper’s cells, there is no evidence of the presence of elementary neurosecretory granules.

Semper’s cells are arranged around a ganglionic centre in a way very similar to the collar cells round the ganglion of the optic tentacle. In spite of this resemblance, in a comparison with the collar cells it is evident that Semper’s cells do not correspond structurally or cytochemically to them, nor to any of the other tentacular cells. Rather than a neurosecretory endocrine complex like that formed by the collar cells, Semper’s cells (or at least some of them) may form a purely glandular endocrine organ.

The cyclic secretory activity of Semper’s cells (Quattrini, 1956; 1957), combined with the fact that their chief inclusions are in the form of lipochondria, is noteworthy in view of recent investigations (Lane, 1963a, d; Meek and Lane, 1964), which have shown that lipid globules in invertebrate secretory and neurosecretory cells possess a complex of diverse enzymes, and may, therefore, play a prominent role in cellular activity. The presence of many lipochondria in an active secretory cell such as Semper’s cell may be further evidence of the importance of lipochondria in invertebrate cells.

I am indebted to Dr. J. R. Baker, F.R.S., for his supervision during the course of my work on gastropod cytology, and to Professor J. W. S. Pringle, F.R.S., for accommodation in his Department. The Huxley ultramicrotome was provided by the Royal Society, and the Akashi electron microscope by the Wellcome Trustees (grants to Dr. J. R. Baker). I wish to thank the National Research Council of Canada; this research was carried out during the tenure of their N.A.T.O. Science Scholarship.

Ashhurst
,
D. E.
, and
Chapman
,
J. A.
,
1962
.
Quart. J. micr. Sci
.,
103
,
147
.
Babor
,
J. F.
,
1895
.
S. B. bohn. Ges. Wiss
.,
34
,
1
.
Baecker
,
R.
,
1932
.
Z. ges. Anat
.,
29
,
449
.
Baker
,
J. R.
,
1944
.
Quart. J. micr. Sci
.,
85
,
1
.
Baker
,
J. R.
,
1946
.
Ibid
.,
87
,
441
.
Baker
,
J. R.
,
1949
.
Ibid
.,
90
,
293
.
Baker
,
J. R.
,
1956
.
Ibid
.,
97
,
621
.
Baker
,
J. R.
,
1962
.
Ibid
.,
103
,
493
.
Beutler
,
B.
,
1901
.
Zool. Jb., Abt
.
2
,
14
,
369
.
Bronn
,
H. G.
,
1928
.
In Klassen und Ordungen des Tier-Reichs. Bd. Ill, Mollusca, Abt
.
2
,
ii
.
Leipzig
(
Akademische Verlagsgesellschaft
).
Campion
,
M.
,
1961
.
Quart. J. micr. Sci
.,
102
,
195
.
Chou
,
J. T. Y
,, and
Meek
,
G. A.
,
1958
.
Ibid
.,
99
,
279
.
Dalton
,
A. J.
,
1960
.
In Cell physiology of neoplasia (M. D. Anderson Hospital and Tumor Institute)
.
Austin
(
University of Texas Press
).
Eckardt
,
E.
,
1914
.
Jena. Z. Naturw
.,
51
,
211
.
Flemming
,
W.
,
1869
.
Arch. mikr. Anat
.,
5
,
415
.
Florey
,
H. W.
,
1960
.
Quart. J. exp. Physiol
.,
45
,
329
.
Gomori
,
G.
,
1941
.
Amer. J. Path
.,
17
,
395
.
Hâckel
,
W.
,
1911
.
Zool. Jb., Suppl
.
13
(Fauna Chilensis 4),
89
.
Hanitsch
,
R.
,
1888
.
Proc. Biol. Soc., Liverpool
,
2
,
152
.
Heath
,
I. D.
,
1961
.
Nature, Lond
.,
191
,
1370
.
Heath
,
I. D.
,
1962
.
Quart. J. micr. Sci
.,
103
,
457
.
Herlant
,
M.
,
1958
.
Arch. Anat. micr. Morph, exp
.,
47
,
1
.
Hoffman
,
H.
,
1925
.
Jena. Z. Naturw
.,
61
,
1
.
Keller
,
W.
,
1902
.
Zool. Jb., Suppl
.
5
(Fauna Chilensis 2),
607
.
Lane
,
N. J.
,
1962
.
Quart. J. micr. Sci
.,
103
,
211
.
Lane
,
N. J.
,
1963a
.
Ibid
.,
104
,
401
.
Lane
,
N. J.
,
1963b
.
Ibid
.,
104
,
495
.
Lane
,
N. J.
,
1963c
.
Ibid
.,
105
,
35
.
Lane
,
N. J.
,
1963d
.
Ibid
.,
105
,
49
.
Lane
,
N. J.
,
1963e
.
Ibid
.,
105
,
61
.
Lawn
,
A. M.
,
1960
.
J. biophys. biochem. Cytol
.,
7
,
197
.
Leydig
,
F.
,
1876
.
Arch. Naturg
.,
42
,
209
.
Luft
,
J. H.
,
1961
.
J. biophys. biochem. Cytol
.,
9
,
409
.
McGee-Russell
,
S. M.
,
1955
.
As in Pearse
, 1954.
McManus
,
J. F. A.
,
1948
.
Stain. Tech
.,
23
,
99
.
Meek
,
G. A.
,
1963
.
J. R. micr. Soc., Proc. Symposium on Cytochemical Progress in Electron Microscopy
,
81
(
3 and 4
),
184
.
and Lane
,
N. J.
,
1964
.
J. R. micr. Soc
.,
82
,
193
.
Mercer
,
E. H.
,
1962
.
In The interpretation of ultrastructure, Symposium of International Society for Cell Biology
, vol.
1
, edited by Harris.
London
(
Academic Press
).
Meisenheimer
,
J.
,
1912
.
Monogr. einheim. Tiere, Bd
.
4
,
Leipzig
,
1
.
Palade
,
G. E.
,
1952
.
J. exp. Med
.,
95
,
285
.
Palay
,
S. L.
,
1958
.
The morphology of secretion. In Frontiers in cytology
, edited by
Palay
.
New Haven
(
Yale University Press
).
Pearse
,
A. G. E.
,
1954
.
Histochemistry, theoretical and applied
.
London
(
Churchill
).
Pelluet
,
D.
, and
Lane
,
N. J.
,
1961
.
Canad. J. Zool
.,
39
,
789
.
Quattrini
,
D.
,
1956
.
Boll. Zool
.,
23
(
2
),
679
.
Quattrini
,
D.
,
1957
.
Ibid
.,
24
(
2
),
243
.
Rhodin
,
J.
, and
Dalhamn
,
T.
,
1956
.
Z. Zellforsch
.,
44
,
345
.
Rotarides
,
M.
,
1929
.
10 Intern. Zool. Kongr., Budapest
,
2
,
952
.
Sarasin
,
P. B.
,
1883
.
Wiirzburg. Zool. Arb
.,
6
,
91
.
Semper
,
C.
,
1856
.
Z. wiss. Zool
.,
8
,
366
.
Shearman
,
D. J. C.
, and
Muir
,
A. R.
,
1960
.
Quart. J. exp. Physiol
.,
45
,
337
.
Simroth
,
H.
,
1876
.
Z. wiss. Zool
.,
26
,
227
.
Simroth
,
H.
,
1891
.
Zool. Jb., Suppl
.
2
, Bd.
5
,
861
.
Sochaczewer
,
D.
,
1881
.
Z. wiss. Zool
.,
35
,
30
.
Tauber
,
H.
,
1900
.
Annu. Mus. Zool. Acad. St. Pétersb
.,
5
,
373
.

Appendix

A summary of the histochemistry of the cells in Semper’s organ in H. aspersa