Explanation of Plate IX (figs. 1-10):

'In all the figures

g.c. = gland cell.

i.c. = ciliated cell.

a.c. = wall of alimentary canal.

typ. = typhlosohle.

y.c. = yellow cells of ecelomic epithelium (chloragoge Zellen).

m.f. = muscular fibres.

nuc. = nucleus.

gran. = secretory granules.

f.g. = ingested particles of fat.

ex. = masses of substance, possibly excretory in nature, found especially in long hunger.

Fig. 1. Transverse section through the gut of Lumbricus at a point where the typhlosohle is complex. The figure is quite diagrammatic and is merely intended to show the general relations of the lining epithelium and the presence and distribution of yellow cells.

Fig. 2. (In this figure and in the following figures, unless special statements are made, the magnification was that effected by Zeiss, Oc. 3, Obj. F.)

Gland cell holding secretory granules and showing basal homogeneous substance (hardened).

Fig. 3. Gland cell, secretory granules dissolved.

Fig. 4 a, b. Taken from different regions of the walls of the alimentary canal of one earthworm. To show variation in the height of the cilia.

c. To show long cilia passino out from block-like end of cells. (Hardened in Flemming's fluid.)

d. To show vacuole in ? badly-noturished ciliated cell, and division of cell substance into internal processes. 

e. Ciliated cell after long hunger. (d and e macerated in 30 p.c. alcohol.)

Fig. 5. From typhlosohle, fasting worm. To show marked ciliation and occurrence of secretory granules in gland cells. (Corrosive sublimate preparation.)' (258-259)

Fig. 1 in text:

'The circular muscle fibres which succeed are again arranged in bundles, and the connective tissue which separates them and which surrounds the abundant neighbouring blood vessels is continuous with that which forms a support for the innermnost layer - the intestinal epithelium. In the cavity of the gut, as I have said above, definite glandular recesses are absent, but through the greater part of its length a well marked involution of the dorsal wall into the intestinal cavity forms the typhlosohle (Pl. IX., Fig. 1. typ.). In this ridge all the layers I have just named are represented; thus it is a highly vascular structure, and yellow cells like those which form the external coat of the gut almost fill its concavity.' (241)

'This intestinal epithelium is, I believe, only one layer in depth. Claparède [note: ' Claparede, Zeit. fur wiss. Zool. Bd. XXXIX'], it is true, speaks of many layers, but his figure is too diagrammatic to allow one to believe that he had seen everything in a section that later methods show. Vejdovsky [note: 'Fr. Vejdovsky, op. cit.'] too, in a figure of Lumbricus, represents small cells as lying at the base of the columnar cells which make up the adult epithelium and evidently regards them as "ersatzzellen." I have not however been able to establish their occurrence at all constantly in Lumbricus. Nuclei indeed are seen besides those which, belonging to the ciliated cells, are placed half-way down the mucous membrane and beside the basal line of nuclei which may be recognized in the gland cells (Fig. 1), but they seem irregular in numnber and occurrence, and I am inclined rather to regard them as belonging to the subepithelial connective tissue or to rare in-wandering leucocytes.' (248)

Figs. 2-3 in text:

'The secretory cells noticed and figured by Vejdovsky [note: 'Fr. Vejdovsky, loc. cit.'] and Benham [note: 'Benham, loc. cit.'] recall the unicellular glands which have already been described in Hydra [note: 'C. Jickeli, Morphol. Jahrb. Bd. VIIL; M. Greenwood, This Journal, Vol. IX.']; they taper slightly towards the internal surface of the gut and more strikingly towards their points of attachment (Figs. 2, 3). These cells display conspicuously, at least at times, a nucleus, cell suibstance and secretory granules; the granules are so numerous in the typically fasting state that it is difficult to realize then the existence of other cell constituents. The protoplasm, when under these conditions it is made evident, is seen to stretch as a supporting framework or spongework throughout the cell; at such times as the granules are less numerous, a basal part of the cell substance, holding the nucleus, is free from them (Fig. 2). When accumulated thus or when it stretches as a temporary network through the cell, the protoplasm shows, I believe, no further obvious structural differentiation. The secretory granules are preserved by osmic vapour, and admirably by corrosive sublimate; they are however more easily broken down by reagents than are the remarkably resistant mucous granules of the unicellular glanids of the skin.' (242-243)

Figs. 4-5 and 7 in text:

''It may be urged, by those who accept the records of these observers, that the minute variations I have described are not incompatible with the received conception of a ciliated cell, or are even artificial and due to want of uniformity in preparation. I must say then that after constant treatment the appearances vary, and that in two other points there is yet a more mainifest deviation from type. In the first place, a vacuole is sometimes present in the external part of the protoplasm (i.e. under the basal band) (Fig. 4 d), while the same region may be collapsed or folded after inaceration so that the cell has a constricted neck. The vacuole is often wanting, however, and the cell substance under the action of hardening reagents, as distinguished from those that macerate, is cominonly dense and homogeneous (Figs. 5, 7, i.c.).' (245-246)

Figs. 4-5 in text:

'there is a comparatively long stretch of intestinal wall in the posterior half of the animal's body where the majority of the cells are in active ciliary movement in the fresh state and where in the hardened specimen the cilia spring from the blocked end of a cell or from a typical basal band (Fig. 4 a, b, c). These are present alone as far sometimes as the level of the ending of the typhlosohle, at other times at this point scattered gland cells are striking. I may perhaps repeat that here and always when glands are present the extreme outer edges of the ciliated cells are apparently in contact laterally in the fresh state, and when hardened show at best only inconspicuous openings between them. Thus the secretion of the cells which lie behind has to reach the cavity of the intestine by narrow channels recalling those by which the mucous glands of the skin open outwards [note: 'J. N. Langley, loc. cit.'] (Fig. 5).' (249)

Fig. 4 in text:

'Cells with obvious cilia. (1) Certain cells, save that they are unusually elongated and branch internally, are not far removed in structure from such ciliated cells as characterize the buccal mucous membrane of Triton, that is their cilia, uniform in length, spring from an apparently hyaline band.

(2) But in many cases the height of the cell is disproportionately increased (Fig. 4 d.), the filiform processes into which its substance breaks internally are very striking and the basal band takes the form of a triangular plug of hyaline substance, the base of the triangle being the apparent point of origin of the cilia while the apex passes off gradually into the protoplasm of the cell (Fig. 4 c.).

(3) And yet again cilia may be traced through the external zone and are only lost when they pass into the protoplasm beyond. With this arrangement the basal band seems sometimes to be made up of rods stouter than the cilia, which look very like their intracellular prolongations; at other times however it is hyaline save for the striae which are the necessary expression of the existence of the perforating cilia. The cilia occasionally bear tiny varicosities before they pass into the body of the cell (Fig. 4 d). Under a sufficiently high power these are distinguishable as belonging each to a ciliary thread [note: 'These varicosities stand singly as a rule, but I think it is possible that each cilium at times bears two, one at the outer limit and one at the inner limit of the basal band.'], and they recall Heidenhain's description of similar thickenings which may be seen under suitable conditions at the base of the intestinal rods of the dog.' (245)

'It will be gathered from the indirect nature of the evidence which I have brought forward that the most convincing proof of the existence of retractile cilia is wanting; I have not seen a hyaline border put out protoplasmic processes, nor have I seers any complete retraction of cilia after death. Variations in length may occur in any one earthworm (Fig. 4 a, b), and in the median region of the typhlosohle, where I think this special form of mobility in the cells is developed characteristically, the cilia, when they show, are finer, more scattered, and less vigorous in action than on the gut walls.' (255)

Figs. 5 and in text:

'It is, I think, indisputable that the typhlosohle is ciliated at times (Fig. 5); the edges of certain cells lining the alimentary canal may, on the other hand, be smooth; now since the typhlosohle has a hyaline border in all well-nourished worms belief in retractility seems inevitable unless we suppose that there is even extraordinary individual variation in structure. It is very probable that when Metschnikoff [note: ' E. Metschnikoff, Zool. Anzeiger, 1878. 1. Jahrgang.'] speaks of the ciliated intestinal epithelium of Microstomum lineare as illustrating a type of cell which has quite lost the power of taking up solid nourishment, he formulates a view which is held widely, yet here in Lumbricus we have cells now ciliated, now crowded with fat. I would urge that these cells are not typical, but rather transitional between the absorptive cells of Hydra with their retractile flagella and the epithelium with its border of rods which is figured in the frog and in the mammal. They are extensile and mobile to a degree which is even surprising when we remember the firm union of many vertebrate tissues, and apparently definite displacement of their substance may occur. Thus there is at different times attenuation or expansion of the outer part of the body of each cell or a vacuole may be present; while not rarely the lateral outlines are so muich obscured that the cells seem confluent in transverse section (Fig. 9). But it is in the border that there is special power of change; the cell would seem indeed to have at its disposal a certain amount of substance which inay form a hyaline band giving origin to fine cilia, or may on the other hand be arranged in rods, stouter, but less readily discernible. And when the cell is especially narrow this changing mass of substance deepens from without inwards; it is wedge-shaped rather than zone-like. '. (235)

Figs. 6-8 in text:

'Cells without obvious cilia. The forms which I place in this group have no motile protoplasmic processes projecting freely into the intestine. In the fresh state the external edges of adjacent cells touch and give the effect of a continuous band, rarely a double band, of hyaline substance (Fig. 8) from which little bullke may be separated off by pressure. In hardened specimens the homogeneity is sometimes preserved (Fig. 7), but as a rule it is possible to apply treatment which brings out certain structural complications. Thus when macerated cells are mounted in fluids of low refractive power it is usual to see that the greater part of the free border is made up of rods set side by side, only a narrow internal zone being still hyaline (Fig. 6 d). The rods remind one irresistibly of Heidenhain's [note: 'R. Heidenhain, op. cit. Taf. 1, figs. V. VI. VII. VIII.']  description of the "Stübehenorgan" in the vertebrate intestine: they are not always equally obvious, nor is their share in the formation of the whole border of the cell constant. Thus at times the internal hyaline band takes up half the depth of the wvhole border; at other times the distinction into two layers is not clear (Fig. 6 b) and the rods are so little separated that the cell seems to have a coarsely striated edge. The inner homogeneous band stains deeply with picro-carmine: the rods stain almost as fairntly as do actuial cilia.' (246-247)

Fig. 6 in text:

'in the great majority of specimens fat is present in large amount and this at once mnarks out the cells which hold it. The fat drops vary much in size, slightly in shape ; the smallest globules lie immediately beneath the free edge of the cell; larger drops are crowded more internally so that protoplasm and nucleus may be obscured, and, apparently passing down, they certainly distend the filiform internial processes (Fig 6 a, b, c).' (247)

'The fat globules which may crowd the typhlosohlar epithelium in some regions, and may be present, though less markedly, in the walls of the alimentary canal, are invariably most abundant in animals taken from rich earth or from leaf mould (Fig. 6 a, b, c). During periods of hunger they disappear, tarrying internally to the nucleuis and apparently passing down the filiform internal processes of the cell which they distend.' (252)

Fig. 10 in text:

'Up to this point, in writing of the gut of Lumbricus I have dealt with epithelium cells and their contents, but there are generally collections of substances between the cells which are both striking and puzzling. These have the form of complex oval masses of matter, and the nature of their constituents is at once variable and obscure. Among these constituents are amorphous clumps of proteid matter (Fig. 10 c.), and irregular particles coloured yellow or orange in the fresh state and staining black with osmic acid, while in some cases there is a pigmented substance which resists the action of caustic potash. I have never demonstrated the presence of nuclei, and indeed if these masses are to be looked upon as cellular, the cells of which they are made up are much changed and disorganized. Fluid is present in varying amount (cp. Fig. 10 d with Fig. 10 b), and the actual shape of any mass is determined by the surrounding epithelium cells, or by the viscosity of some of its proper constituents.' (256)

Explanation of figs. on Plate XIII:

Fig. 1. V. S. cornea of albino rabbit, hardened in a mixture of chromic acid aind spirit; a, corneal epithelium; b, cornea proper; c, elastic lamina, posterior epithelium removed; d, clear hem on corneal epithelium.

Fig. 2. a, various forms of cells from lowest layer of columnar epithelium isolated by iodised serum; b, similar cells; c, bases of a and b; d, 'digitate' cells from higher layer; e, prickle cells further up towards surface of cornea.

Fig. 3. Peculiar appearance sometimes seen in vertical sections of cornea of ox; a, tall compressed cells; b, similar cells swollen out and compressing a.

Fig. 4. Corneal epithelial cells of ox isolated by iodised serum; a and b, lowest layer of cells; b. apex grasped by fangs of a digitate cell; c, ' digitate' cells; d, smaller digoitate cells higher up; e, prickle cells.' (338)

Fig. 1 in text:

'On examining a vertical section of a cornea which h, as been stained with picro-carmine and mounted in glycerine, we get such a view as is shown in Fig. 1, Pl. XIII., where a represents the epithelium covering the anterior surface of the cornea proper, b; c the posterior elastic lamina. The picro-carmine has stained the nuclei of all the cells red, their substance yellow, and the cornea proper and elastic lamina red.' (335)

'Fig. 1 d is very instructive, for it shows a part of the corneal epithelium detached from the cornea proper, each cell showing its clear hem.' (336)

Fig. 2 in text:

'The columnar cells forming the lowest layer are not all of the same height, nor of the same size, as generally represented. This is well brought out in Fig. 2, a and b, and Fig. 2 c shows the bases of these cells, and here we note that they are not all of the same size at the base...

The lowest columnar cells are always slightly expanded at their bases, and the clear band is not so marked as after hardening. Their appearance is shown in Fig. 2 a, taken from a preparation isolated by iodised serum. From the middle layers a few prickle cells are always obtained of various forms, and Fig. 2 b shows the difference in the height of these cells. From above this layer we have succeeded in isolating smaller cells-digitate cells, whose teeth fit in between and grasp the apices of several columnar cells. We shall allude to these more fully presently. Prickle cells can also be easily isolated, Fig. 2 e.' (336)

Fig. 3 in text:

'Fig. 3 gives a view of a section of the cornea of the ox... but certain of the appearances however are peculiar and apt to lead one into error, unless controlled by examining other preparations. Every here and there groups of short broad cells (b) are to be observed, occurring between groups of tall cells (a). This is not a normal appearance but due to the swelling out of those at b compressing those at a, and thus giving rise to this peculiar appearance, which we have also seen in the rabbit's cornea.' (337)

Fig. 4 in text:

'We find that Rollett [note: 'Rollett, article Cornea in Stricker's Histology, English Edition, p. 424.'] has also noticed in the lowest layer of cells of the cornea of some animals a basal 'hem' or 'border'... He says, "The expanded basal borders of the cells are so applied to each other, or are so superimposed upon one another by their thin edges, that the borders of the several cells seen collectively in situ make a bright stria which forms the line of demarcation between the epithelium and the corneal tissuet." [note: 'L.c. p. 427.'] This is also the view we take of the matter, for the base of each cell is always expanded somewhat and broader than the part immediately above the base (Fig. 4, a and b).

We now proceeded to isolate the cells from the cornea of the ox by means of iodized serum. Fig. 4, a and b, shows the lowest layer of columnar cells. They are very remarkable on account of their being so elongated; they are as elongated as any epithelium in the body .Invariably their base is flat as described by Prof. Cleland [note: 'On the Epithelium of the Cornea of the Ox, Journal of Anat. and Physiology, Vol. II. P. 362.']... The clear hem is very delicate and not nearly so pronounced as in the rabbit. Above these cells Prof. Cleland in 1867 described cells of peculiar shape, to which he gave the name 'digitate'. "They are of irregular shape, about twice as broad as the columnar epithelium, but by no means so elongated." He describes them as " rounded and even in outline at the superficial extremity," (but they are not all so (Fig. 4 c), though some are,) and "jagged at the other, and send in processes or 'digitations', which may be three or four in number, and which appear to fit in between the tapering points of the columnar cells." We have directed especial attention to these peculiar 'digitate' cells, for their existence has been denied by Rollett [note: 'Op. c. p. 424'], but there cannot be the slightest doubt of their existence... They present much the appearance of a molar tooth with its fangs, dipping in between the apices of the columnar cells, and Fig. 4 b shows how they are placed, each cell being in relation with the apices of several columnar cells.' (337-338)

'Above this layer of digitate cells we have a stratum of smaller cells some of which are 'digitate' (Fig. 4 d), and there is a gradual transition between them and small prickle cells (which occur in very large numbers in the cornea of the ox and sheep), and gradually as we pass upwards we come to the large flat squames on the surface.' (338)

Explanation of Plate VII, figs. 1-9:

'Fig. 1. Diagram of the Cardiac apparatus and gills of Octopus, with the cardiac and respiratory nerves.
Vc. = Vena Cava. KY. = Kidney Vein. BH.= Branchial Heart. G. = Gill. LA. = Left auricle. RA. = Right auricle. V. = Ventricle. CA. = Cephalic Aorta. Ga. = Genital artery. Va. = Visceral artery. SF. = Veins from sides. P. = Pleural ganglia. S. = Stellate ganglia. Vg. = Visceral nerve. C. = 1st Cardiac Ganglion. Cr. = 2nd Cardiac Ganglion. B. = Branchial Ganglion.

The dotted lines indicate the parts covered by glands.

Fig. 2. a. Muscle fibre of the ventricle of Octopus. (x 500.) Alcohol and Hematoxylin.

Fig. 2. b. Portion of a muscle fibre of the ventricle of Sepiola. (x 500.) Gold chloride.

Fig. 3. a. and 3. b. Plasma cells from the julletion of the kidney and auricle of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 4. Ganglion-cell from the cerebral ganglia of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 5. Plasma cell from the cutis of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 6. Connective-tissue cells; a. from the auricle, b. from the cutis of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 7. Diagram of the branchial and cardiac nerves of Aplysia.

LV., RV. = Left and Right Visceral nerves. Vg. = Visceral ganglion. Gen. = Genital nerve. O. = "Olfactory organ." rn. = Left branch of the nerve to the gill. nb. = Branch to the pericardium. G. = Gill. P. = Pericardium. A. = Auricle. V. = Ventricle. a. = Aorta.

Fig. 8. a. Plasma cell on auricular muscle of Helix.

Fig. 8. b. Plasma cell from the connective tissue membrane between the loops of the genital duct of Helix. (Zeiss. D. Oc. 4.)

Fig. 9. Diagram of the cardiac nerves of Helix.

SO. = Supracesophageal ganglia. Sub. O. = Subcesophageal ganglia. Lv. = Left visceral nerve. Os. = Ovisperm duct. K. = Kidney. P. = Pericardium. A. = Auricle. V. = Ventricle. a. = Aorta.' (338-339)

Fig. 1 in text:

'The species chosen as most convenient for study was the common Poulp, Octopus vulgaris, and to it the following pages will be almost exclusively confined.

The cardiac apparatus of Octopus (Fig. 1, Plate VII.) consists of a pre-branchial or venous, and a post-branchial or arterial section. The former collects the blood from the veins and drives it through the gills; the latter receives the aerated blood fromn the gills and propels it through the bodv. The former section may be divided into the following parts:-

Vena Cava.

Kidney Veinis.

Branchial Hearts.

The latter consists of :-

Auricles.

Ventricles.

The Vena Cava (Fig. 1. VC.) is a straight thlin-walled tube, which runs down the ventral side of the liver, parallel to the intestine, towards the heart.

At the level of the front end of the ventricle it divides into two branches, which being covered with glands considered to have a renal function, may be conveniently distinguished as the Kidney Veins (KV.)

Each Kidney Vein, curving out towards a gill, is continued into a bulged oval body known as the Branchial Heart (BH.), their cavities being separated by a pair of valves at the junction. The Branchial Hearts have been usually held to be muscular organs devoted to propelling the blood through the gills; but that this view is not entirely correct will be shewn by the description of their minute structure given below. From each Branchial Heart a thick-walled "branchial artery" leads to the gill (G). The efferent vessels of each gill form a number of veins which soon unite to form a tuLbular auricle (LA. RA.), thin-walled but stouter than the Vena Cava. The two auricles open into the medianly situated, fleshy, somewhat globular ventricle (V), from the cavity of which the blood is prevented from returning by a pair of valves at each auriculo-ventricuilar orifice. 

Three efferent vessels issue from the ventricle, the most important being the great cephalic aorta (CA.), which starting from the posterior dorsal right region of the ventricle curls round to run to the head. Medianly and posteriorly is given off a small genital artery (G a.); and from the anterior ventral edge a small bulb gives rise laterally to a fine artery to each auricle and to a somewlhat larger visceral artery (V a.) medianly to the intestine.' (263-264)

'The whole cardiac system is supplied by a pair of nerves from the pleural ganglia in thie head... Their relations to the neighbouring organs and directions for finding them have been accurately given by Fredericq... Their branches and ganglia need however a more detailed description (Fig. 1). On issuing from the skull, each nerve appears as a double cord from which various small branches are given off. The cords on either side end in a small ganglion lying under the liver, from which go nerves to the body wall and the columnar muscle inserted into the mantle, while a single main trunk is continued downwards towards the heart. Just in front of the auricle it dilates into a ganglion, which may be called the 1st Cardiac Ganglion (C). From this issue a fine nerve to the generative duct, a nerve which enters the auricle, gives off a branch there and then passes through to the ventricle, and lastly a stout nerve which runs dorsally to the auricle down to the branchial heart, where it is connected with a ganglion-the 2nd Cardiac Ganglion (Cr). From this ganglion go nerves to the substance of the branchial heart and a short way into the kidney vein, but the main trunk proceeds to the gill, at the base of which it expands into the Branchial Ganglion (B). No other ganglia are revealed by dissection on the visceral nerves or their branches...

From the pleural ganglia there also runs on either side a stout nerve ending in tlle stellate ganglion (s) and containing the motor fibres for the mantle; so that the gills and heart are connected by nervous structures with the motor organs of respiration.' (267-268)

Fig. 2 in text:

'The fibres of the ventricle are of an elongated fusiform shape, with long tails, but are shorter and thicker than those of the auricle. Like the others they have no sarcolemma. Thus far they resemble the ordinary smooth muscle cells of vertebrata, but in addition they possess a fine but regular transverse striation very like that of the branchial heart, the fibres of which they closely resemble. Careful focussing also shows in the fibre indications of a granular core of a different nature to the outer zone. Both of these features are best seen in osmic acid or alcohol preparations (Fig. 2a) [note: This axial column of apparently less differentiated substance has been noticed by most observers of molluscan muscles. Thus Ranvier (Traite technique d'Histologie, p. 851), figures a gold chloride preparation of the retractor muscle of Helix pomatia showing distinctly the "cordon protoplasmique"; Leydig and Kölliker saw it in the buccal muscles of Gasteropoda, and H. Muller and F. Boll in the branchial hearts of Cephalopoda. (See F. Boll, Arch. f. mikr. Anat. Bd. v. Suppl. Hft. p. 28).'].' (266)

'while the ventricular fibres of Eledone resemble those of Octopus, those of Sepia present a bolder striation, and in Sepiola the difference is still greater. In this animal the fibres are much larger, and the average distance between the striations is 3.3m.; the dark bands are very sharp, but narrow; and the margin of the fibre is bulged out opposite each broad clear disc (Fig. 2. b). The central core is much more distinct than in Octopus, occupying about one third of the diameter of the fibre, and staining very deeply with gold chloride, while the outer zone remains clear. It also shows a beaded outline corresponding with the striations. [note: 'Transverse striation has also been noticed - chiefly in hearts or the buccal mass - by many observers, a list of whom is given by Boll (loc. cit.) and by F. Darwin (Journ. of Anat. a. Phys. Vol. x. Part Ii. April 1876, p. 506). Dogiel (Arch. f. mikr. Anat. Bd. 14) has also seen it in various hearts, (Pecten, Anodon, Aplysia, Helix) and Haller (Gegenbaur's Jahrbuch, 1883) in the hearts of Fissurella and Haliotis. Margo (Wien. Sitzusngsb, Bd. 39) has described the black bands in the striated shell muscle of Anodon as doubly refracting.'].' (266 [note 266-267])

Figs. 3-5 in text:

'Two kinds of elements however occur [in Pterotrachea] which might be mistaken for nerve cells, and which I believe have been taken for such in other Molluscs. The first of these are large, moderately granular, often roundish or oval cells, with a not very distinct oval nucleus placed near the periphery of the cell, and in which a nucleolus is sometimes visible. A distinct capsule is not present. These cells occur in great abundance on the walls of the rhythmically contractile kidney, and a few are usually to be found in the auricle [note: 'Although in teased preparations of auricle these cells were often found, I am inclined to doubt whether they occur normally in the auricle itself, and to think that in these cases they have appeared from adhering fragments of the closely connected kidney, which owing to the transparency of the tissues it is difficult to cut clearly off at the boundary. In preparations where the auricle was cut through at a distance from the kidney no such cells were found. At the same time it is possible that they may occasionally wander from the kidney to the auricle. They are also found in the pericardial wall.']. I have not observed them in the ventricle. Although the approximately oval form (Fig. 3 (a) Pl.VII.) is perhaps the commonest shape, yet it is by no means constant. In many cells short blunt processes or pseudo-podia are seen (b) which suggest the power of amoeboid movement, and occasionally a single such process may occur, causing some resemblance to a ganglion cell with the stump of a nerve fibre. The number and variations of the processes however oppose this idea, and a comparison of such a cell with a true ganglion cell (Fig. 4) at once shews a number of differences in appearance. Further these cells are never found in connection by fibres with either nerve or muscle, and are shewn to be very loosely applied to the tissues by the fact that in teased preparations large numbers of them become entirely detached and float like the blood corpuscles in the fluid in which they are mounted. Lastly, they occur not only in the parts mentioned, but in the connective tissue all over the body, as is well seen in preparations of the cutis, where they (Fig. 5) abound. These facts point to the conclusion that they are wandering connective tissue cells; and we may almost certainly identify them with the "plasma cells" which have been shewn by Brock [note: 'J. Brock. " Untersuch. ii. d. interstitiellen Bindesubstafiz der Mollusken." Zeit. f. Wios. Zool. Bd. 39. 1883.'] to be so characteristic of the connective tissue of some other Molluscs (Aplysiadae, Pulmonatae).' (321-322)

Fig. 6 in text:

'The second form of element which might be considered nervous [in Pterotrachea] is of much smaller size and more delicate structure. These cells again I found only where connective tissue was abundant. They are granular branched cells with a usually indistinct nucleus. Occasionally the branches may be reduced to two, and there then appears some resemblance to a small bipolar ganglion cell (Fig. 6 (a)). These cells however are nothing more than the ordinary connective tissue cells, which give off processes to form fibres and are found everywhere throughout the connective tissue (Fig. 6 (b)). A comparison of the two figures makes plain the identity of those of the heart with those of the cutis. These also are described and figured by Brock in the connective tissue of Aplysia as stellate connective tissue cells, and very similar cells by Haller as occurring 'in small quantity in the auricle of Fissurella and as forming a second type of ganglion cell.' (323)

Fig. 7 in text:

'Apart from the improbability of the existence of apolar ganglion-cells in the heart, the incorrectness of Dogiel's drawing of the heart and the adjacent nerves causes great doubt as to the accuracy of his interpretation of these cells.

In his figure (Taf. V. a. Fig. 15) of the "branchial" ("visceral," Spengel) ganalion he entirely overlooks the olfactory organ to which the right nerve runs, and he represents the left nerve as of equal thickness and as running straight to the auricle. But instead of such a short thick left nerve, there come off from the left half of the double "visceral " ganglion two distinct long nerves. Of these one supplies the generative duct, while the other runs on to the gill. Here it divides into two, the left one running on straight towards the hind end of the gill while the right curls round anteriorly and appears to end in the pericardium near the origin of the auricle (Fig. 7).' (325)

Fig. 8 in text:

'In the auricle of the Snail may be found upon the muscle bundles [note: 'Both the auricle and ventricle are formed of a meshwork of bundles of striated muscle fibres, that of the ventricle being denser and thicker.'] a number of plasma cells which are, there is little doubt, identical with Dogiel's "apolar!" ganglion cells. Some of these cells of an oval or pear-shaped form present a considerable resemblance to ganglion cells, and if only these were observed might be considered such. But when others of varying shapes, and others in process of division are seen, such an idea becomes untenable; and when the striking similarity of these cells with the plasma cells of the connective tissue is noticed, no doubt can remain of the identity of the two (Fig. 8).' The various stages of division also which are met with, from a group of two or three contiguous cells to a nest of closely packed small ones, point clearly to the true nature of these elements. True ganglion cells I believe do not exist in the Snail's heart.' (327)

Fig. 9 in text:

'The heart... receives nerves both at the auricular and aortic ends [note: 'A similar double nerve supply has been described by Haller, loc. cit. as existing in Muricidae and Fissurella.']. (Fig. 9.) The discovery of this nerve to the heart at once necessitated a reconsideration of the results obtained by previous workers, and the question of its function became important.' (328)