d. The vitreous body is not a homogeneous secretion, but is composed of

prisms of refracting substance, each with a denser central fibre.

Let us go over these four points in detail.

(a) As to the first, the question whether there is an alternation of pigment and visual cells, I am not prepared as yet to make a positive statement, since my not seeing both kinds as they are described has little evidential value against the fact that Claus and Schewiakoff both claim to have seen them. Perhaps proof could be obtained one way or the other by maceration of fresh or of specially prepared material, which none of us had. My evidence for not confirming alternation rests wholly upon sections. Fig. 58 represents a radial section through part of the larger eye of Charybdea, made from an osmic preparation which in this case showed two advantages over the material fixed in corrosive-acetic (usually by all odds the best), namely, that the vitreous body (_vb_) was not shrunken away from the retinal cells, as almost invariably happens, and that the retinal cells were contracted apart from one another in some places in such a way as to be almost equal to a macerated preparation. Now, in the figure it is seen that there is an apparent alternation of two kinds of cells, more regular than I usually find, but the ones that are undoubtedly the pigment cells of Schewiakoff are the ones that show the fibrous processes like his visual cells, and the pigment streaks in the vitreous body are seen to be integral parts of the cells, not cone-shaped masses lying in the vitreous body, merely associated with the pigment cells. If these _are_ the pigment cells of Schewiakoff, the shorter cells in between must be his visual cells, yet they can by no means be said to conform to a spindle-shaped type, nor are their nuclei always at a lower level than (that is, internal to) those of the pigment cells. If the long cells with the fibres are, on the other hand, considered the visual cells of Schewiakoff, then again we find nonconformity to a spindle-shaped type, and nuclei not always at a lower level. The matter of alternation of nuclei at different levels seems to me any way too slight a distinction upon which to base a difference in function. It is a necessary mechanical consequence of the crowding together of many cells on one surface. And in many cases in perfectly radial sections through the retina I find the nuclei fewer in number and arranged in very nearly a single level. The retina of the smaller eye represented in Fig. 69 shows this. In sections further along in the same series the nuclei are found at different levels, due without doubt to the slanting cut.

EXPLANATION OF LETTERS IN TEXT FIGURE.--_C_--concretion cavity; _CO_--cornea; _CP_--capsule of lens; _CSC_--cavity of sensory club; _EC_--ectoderm; _EN_--endoderm; _ENC_--endoderm of sensory club; _L_--lens; _NC_--network cells; _NF_--nerve fibres; _RT_--retina; _SLA_--supporting lamella; _VF_--vitreous body.]

Fig. 72 is a horizontal section through the large eye, and shows that here, too, when the sections pass through the eye just radially, the nuclei are not found at different levels sufficiently definite to suggest two kinds of cells.

In the inner corner of the retina in the same figure (69) are seen cells without pigment which show nuclei undoubtedly at different levels. These cells in this position are a regular feature in the retina of the smaller eye. Schewiakoff considers them purely visual, because of the lack of pigment. In so doing it seems to me he forgets his own standard for discriminating between pigment and visual cells. The pigment cells of the retina, according to him, are the same thing as the cone-shaped supporting cells found elsewhere in the nervous epithelium, and are, therefore, distinguished from the visual cells primarily by shape and by position of nucleus, secondarily by the greater development of pigment. When on the ground of pigmentation alone he calls the cells in the corner of the retina visual, he judges them by only the second test, and in so doing virtually admits, as it seems to me, that shape of cell and position of nucleus are matters of no great moment. His own standards place him in a dilemma. If on the other hand he judges by the lack of pigment, the cells are visual; if by shape of cell and position of nucleus, they are both visual and pigment cells without the pigment or supporting cells. What use there would be for simple unpigmented cells in one limited region of the retina is hard to see, so he naturally takes the other horn of the dilemma and calls them visual because they have little or no pigment.

The distinction, then, between pigment and visual cells is brought down to one of pigmentation only. Schewiakoff’s test for this is that in the visual cells “Das Pigment durchsetzt aber nicht das ganze Protoplasma des centralen Zellenabschnittes, sondern ist auf seine Oberfläche beschrankt (Fig. 19, _sz_), so dass der innere, axiale, stark lichtbrechende Theil vollkommen frei von demselben ist.” (’89, p. 37.) That is, in a section through the ends of the retinal cells each pigment cell will appear as a uniformly pigmented area, while each visual cell will appear as a light, strongly refracting spot with a ring of pigment around its periphery. This is the arrangement given in his Fig. 19.

An arrangement so definite ought to be easily made out in sections, yet I have not been able to find it so. My sections show considerable difference in the amount of pigmentation even in material preserved with the same killing agent. If the retina is heavily pigmented the ends of the cells have the appearance shown in Fig. 62, which represents a portion of a cross-section. The ends are seen as clearly defined polygonal areas differing among themselves in size, but not showing two types of size, or two kinds of pigmentation, the one uniform, the other a ring of pigment around a highly refracting central portion. If the retina is but slightly pigmented--and some were so light as to make depigmentation unnecessary--a difference is seen in the pigment, as shown in Fig. 63, but in no case were areas found that showed a highly refracting centre surrounded by a ring of pigment. (The unexplained structures in Fig. 63 will be referred to a little later.)

Figures 59-62 are a series of four successive sections drawn with the camera lucida for comparison with Schewiakoff’s Figs. 20 and 19, and to show that the presence of two types of cells plainly marked within the retina by the position of the nuclei at different levels is at least not clearly demonstrated. Only the nuclei are drawn, since the cell bodies are not easily distinguished from the surrounding fibres. The eye is the same as that from which Fig. 72 was made. Fig. 59 shows a relatively small number of nuclei of slightly larger size than usual. These I take for two reasons to be nuclei of the ganglion cells that are found in the fibres at the base of the retinal cells (Figs. 58, _gc_, 69 and 72). They are the first nuclei struck in tracing sections toward the retina, and in the series from which Fig. 58 was taken similar nuclei appeared in both transverse and radial cuts through the retina stained brightly and clearly with hæmatoxylin, whereas the nuclei of the retinal cells proper were stained a diffuse brownish-yellow from pigment that had evidently gone into solution. Fig. 60 shows the closely aggregated, smaller nuclei of the retinal cells surrounded by the nuclei of the outlying ganglion cells. Schewiakoff’s corresponding drawing (’89, Fig. 20) shows at this level a definite alternation of the bodies and nuclei of unpigmented visual cells, with the smaller, pigmented, proximal processes of the pigment cells. In the next section (Fig. 61) the pigmented ends of a few of the cells have been struck, and the following section (Fig. 62) shows that, in this heavily pigmented specimen at least, there is no good evidence within the retina itself of two kinds of cells, so that it is apparent that at any rate we cannot accept Schewiakoff’s conception of the structure.

(b) Yet the fibres that Schewiakoff observed and associated with special visual cells occur beyond question. Fig. 64 is a drawing of the first cut through the vitreous body of Charybdea, and in among the sections of the pigment streaks are seen sections of processes lying within clear spaces exactly as Schewiakoff figures his visual fibres (’89, Taf. II, Fig. 18). That the fibres occur is indisputable, but as to the cells to which they belong I can say nothing except that from such evidence as I have given in the preceding paragraph I conclude that they come from pigmented retinal cells of not very different type within the retina from the others, if different at all.

(c) On the third point, that the pigment streaks in the vitreous body belong to underlying cells and are continued distally into fibrous processes like the visual fibres of Schewiakoff, the evidence is decisive. Fig. 58 has already shown it, and if this were not enough, a case of unusual stoutness of the fibres drawn in Fig. 67 is conclusive. The preparation from which the section is taken was one preserved with corrosive-acetic, and I have drawn the outlines with the camera in order to avoid exaggeration of the fibres as far as possible, and also to show the shrinkage of the vitreous body (_vb_). It is the shrinkage of the vitreous body that makes it so difficult to determine the exact relation of structures seen in the vitreous body to the retina. The fibrous processes run through the vitreous body to the “capsule” of the lens (_cp_) (see also Fig. 72), a layer of homogeneous substance much resembling that of the vitreous body, which is classed as a part of the vitreous body, but usually in the shrinking adheres to the lens. The capsule is therefore regarded by Schewiakoff as a secretion of the lens cells. Some fibres were found by him to have the appearance of branching upon reaching the surface of the capsule, others of passing through it and of seemingly ending among the cells of the lens. The same appearances were given in my sections. It is altogether impossible in the distal portion of the vitreous body to distinguish between the fibres of Schewiakoff and those that come from the long pigment cells. (Figs. 64-66 represent the appearance of the vitreous body at successive levels, and are from the same series of sections as Figs. 59-62 and 72.) In Fig. 64 the sections of the processes that Schewiakoff calls visual are easily distinguished from the sections of the long pigment cells. In Fig. 65, which is two or three sections nearer the lens, the pigment cells are shown by their cross-sections to be tapering down, and in Fig. 66, nearer still to the lens, the two kinds of processes are no longer to be distinguished from each other. In a few cases I have found pigment in a fibre which but for this would be called one of the visual fibres of Schewiakoff. Such considerations as these, the similar appearance in cross-section, the finding of pigment in a few cases, and the inability to trace to any readily distinguished special type of retinal cell, make me wonder whether the visual fibres of Schewiakoff are anything more than the distal processes of pigment cells, into which the pigment granules happened not to be produced at the moment of fixation.

Fig. 63, however, where the retina was only slightly pigmented, rather speaks against this view, for the number of darkly pigmented areas seen here (which are shown beyond question by radial sections to belong to the long pigment cells) is not great enough to account for the number of both pigment areas and visual fibres of Schewiakoff seen in such a section as Fig. 64. This would throw the visual fibres of Schewiakoff back upon some of the slightly pigmented cells of Fig. 63, otherwise not distinguished. I think the question cannot be settled without the maceration of fresh material, and experiments upon eyes killed in the light and in the dark.

In such cases as that of Fig. 63 it would seem conclusively shown that the long pigment cells must belong to a different type from the short, but as I have already said I can find no regularity in either their shape or in the position of their nuclei. And on the other hand Fig. 58 shows that the reverse relation may obtain and the long cells be less deeply pigmented on the edge of the retina than their shorter neighbors, so that it looks as if all the short cells had to do was to project half their pigment out into the vitreous body in order to become exactly like the long ones. This they could do if, as is possibly the case, they are prolonged into “visual fibres” of Schewiakoff that have escaped observation and so do not appear in the drawing.

Fig. 58 shows one more thing that is worthy of remark in passing. In the preparation in which the vitreous body (at this point at any rate) was not shrunken away from the retina, the fibre from each long pigment cell does not lie in a clearly defined space or “canal,” such as is usually described as a constant structure of the vitreous body. Very likely these canals are formed only by shrinkage around the fibres, and the irregular shape of the spaces around the three fibres in Fig. 67 rather bears out the same supposition.

As to the structure of the vitreous body, apart from the fibres and pigment streaks already mentioned, I find it to be made up of prisms extending from retina to capsule of lens, each containing a central axis or fibre. Fig. 64 shows that the space around the pigment areas and “visual fibres,” instead of being homogeneous, is wholly filled with the polygonal cross-sections of these prisms. In Charybdea they are generally more difficult to perceive than in my best material of Tripedalia which was killed in acetic acid. In this the polygonal areas stood apart from each other more plainly. Curiously enough I have been unable to demonstrate in Tripedalia the “visual fibres” of Schewiakoff. Here and there were found spaces that at first sight reminded of them (Fig. 69, _sh_), but they contained no central fibre, and were probably due to shrinkage. The polygonal areas themselves, however, often contained a clear spot in the centre, at one side of which would be found the cross-section of the fibre, as is shown in many cases in Fig. 68. The clear spot is here undoubtedly due to shrinkage of the gelatinous substance of the prism.

I think that these prisms and fibres are the direct continuations of retinal cells. In a section such as that drawn in Fig. 63, which takes just the very tops of the cells of a slightly pigmented retina, in the centre of the section just grazing the space that lies between the retina and the shrunken vitreous body, most of the cells toward the middle (where especially the extreme tips are taken) show in their centres a dot exactly corresponding to the dots in the polygonal areas of the vitreous body. In the exact middle of the section, where only the cell walls appear, slightly indicated, a dot is seen in each case. The size and shape of the ends of the cells correspond with those of the polygonal areas in the vitreous body, and I do not doubt that the latter are continuations of the former. The vitreous body, then, instead of being homogeneous, is composed of the clear highly refracting outer ends of retinal cells. The assumption lies near that these are the true visual rods, but of course it is assumption only.

To give a brief review, the points in which my conclusions differ from those of Schewiakoff are as follows: I find (1) that the long pigment streaks are parts of retinal cells continued into processes like his visual rods; (2) that the vitreous body is composed of prisms with central fibres proceeding from retinal cells; (3) that I am unable to get satisfactory evidence of two types of cell distinguishable within the retina, and at any rate find considerable evidence against the two types he distinguishes.

These results are not wholly satisfactory, for they leave us with three kinds of fibrous processes in the vitreous body which for the present we are unable to trace to three, or even two distinguishable types of cell in the retina. It would be more pleasing if we could confirm Schewiakoff’s simple conception of the structure, with its one set of visual rods in the vitreous body referable to a clearly marked type of sensory cells in the retina, but I think the evidence that has been brought up justifies the conclusion that in some respects he saw too much, in other respects too little. This is not to be wondered at, since his material, to judge from a single statement, consisted of but twelve marginal bodies, and, moreover, the work on Charybdea forms but one portion of a paper that is excellent for the clearness of its descriptions and illustrations.

Before leaving the subject I must mention that Wilson suggested from his observations on Chiropsalmus that the vitreous body had a prismatic structure, but he was probably mistaken when he thought he found evidence of nuclei in it. Claus says that the retina is composed of pigment and rod cells alternating, and Wilson agrees with him, but under a sketch of a sense cell from the nerve he makes the express statement “not very well preserved.” It seems very probable, therefore, that he followed Claus’s interpretation rather than independent observations, and Claus interpreted his results very much by analogy of what had been found in other forms.

The smaller complex eye which is represented in Fig. 69 agrees in structure very closely with the larger. The chief differences are that sections do not show pigment extending into the vitreous body, that there is no “capsule” to the lens, and that the lens seems to be supported by a kind of stalk formed by a thickening of gelatine of the supporting lamella (_sl_). The gelatinous thickening lies between the lens and an outgrowth of endodermal cells (_en_) from the canal of the club. This outgrowth is a constant feature, figured by Claus and Schewiakoff for Charybdea, and by Wilson for Chiropsalmus, and found in Tripedalia also. The regularity of its appearance in all three genera leads one to suspect that it may have some significance not yet understood.

Just above the smaller eye there lies a mass of cells of peculiar structure (Fig. 69, _nc_). They are of a rounded polygonal contour, with a comparatively small circular nucleus in the centre, and are found in this region only. In and amongst them bundles of fibrous tissue are found in the sections, which pass from the surface cells to the supporting lamella. Claus describes the contents of these cells as coarsely granular protoplasm and says they cannot be taken for ganglion cells. He is inclined to believe that they play the part of a special supporting tissue. Schewiakoff, on the other hand, is convinced that they are ganglion cells, and finds processes passing out from them (’89, Taf. II, Fig. 22). I find, however, that the cell contours are perfectly regular and clearly without processes, and it is incomprehensible to me how, if his material was at all well preserved, he could for a moment have taken them for the same thing as the big multipolar ganglion cells with large nucleus and nucleolus which lie in about the same region and were correctly described and figured by Claus but are not specially mentioned by Schewiakoff. I cannot agree with Claus, however, that their contents are composed of coarsely granular protoplasm. That which appears such by low magnification shows itself under high powers to be a beautiful network with thickenings at the nodes of the meshes, which is brought out very plainly by a cytoplasmic stain such as Lyons blue. Around the nucleus is seen a more or less well-defined clear zone. What the function of the cell is remains as unknown to me as to Claus and Schewiakoff.

There is left one more point in reference to the nervous system upon which I wish to say a word. Claus and Schewiakoff both describe the wall of the crystalline sac as structureless, formed by the bare supporting lamella. The credit is due to H. V. Wilson of finding in Chiropsalmus that it has a special lining of epithelial cells, which he figures as a continuous, flattened layer. In both Charybdea and Tripedalia I find traces of the same in nuclei here and there, but whether they are the remains of a once continuous layer or not the sections do not show satisfactorily.

This ends the account of what it seemed worth while to say at present upon the nervous system. In concluding, the writer wishes to express his thanks for the help afforded by Dr. Wilson’s notes, in particular on the subject of the vascular lamellæ, and desires to make especial acknowledgment of his indebtedness to Professor Brooks, whose suggestions, based upon many years of experience with the Medusæ, have been most welcome and helpful, and whose evidences of unfailing kindliness, both in Jamaica at the time the material was obtained and in Baltimore when it was being studied in the laboratory, take a most honored part in the pleasant memories associated with the work.

LITERATURE REFERRED TO.

CLARKE, H. J. ’78. Lucernariæ and their Allies. Washington: Smithsonian Institution.

CLAUS, C. ’78. Ueber Charybdea marsupialis. Arb. aus d. Zool. Inst. d. Univ. Wien, Band II, Heft 2.

DOFLEIN, F. ’96. Die Eibildung bei Tubularia. Zeitsch. f. wiss. Zool., Bd. LXII, Heft 1.

HAECKEL, E. ’79. Das System der Medusen. Jena.--’81. Challenger Report on the Deep-sea Medusæ. Vol. IV.

HERTWIG, O. and R. ’78. Das Nervensystem und die Sinnesorgane der Medusen. Leipzig.

HESSE, R. ’95. Ueber das Nervensystem und die Sinnesorgane von Rhizostoma Cuvieri. Zeitschr. f. wiss. Zool., Bd. LX, Heft 3.

MÜLLER, F. ’59. Zwei neue Quallen von St. Catherina (Brasilien). Abhandlungen der naturf. Gesellschaft zu Halle.

SCHEWIAKOFF, W. ’89. Beiträge zur Kenntniss des Acalephenauges. Morph. Jahrb., Bd. XV, Heft 1.

WILSON, H. V. Unpublished notes.

TABLE OF REFERENCE LETTERS.

_afr_ = adradial furrow.

_afr´_ = furrow in Tripedalia that separates perradial from interrad. regions in lower half of bell. (In Charybdea the same furrow is directly continuous with _afr_.)

_ax_ = axis of nerve.

_c_ = concretion.

_cc_ = canal underneath _ivl_, connecting the two adjacent marginal pockets.

_ccl_ = circular canal.

_ci_ = cilia.

_cm_ = circular muscle.

_co_ = cornea.

_cp_ = capsule of lens.

_cs_ = covering scale of niche.

_csc_ = canal of sensory club.

_ct_ = canal of tentacle.

_ct´_ = beginning of canals of lateral tentacles in Tripedalia.

_ec_ = ectoderm.

_ece_ = ectoderm of exumbrella.

_ecs_ = ectoderm of subumbrella.

_ed_ = distal paired eye.

_el_ = larger unpaired eye.

_en_ = endoderm.

_enc_ = endoderm of sensory club.

_enf_ = tract of nerve fibres underlying endoderm.

_enfl_ = endoderm of floor of stomach.

_enp_ = endoderm of stomach pockets.

_enr_ = endoderm of roof of stomach.

_ens_ = endoderm of stomach.

_ep_ = proximal paired eye.

_es_ = smaller unpaired eye.

_fc_ = funnel leading into canal of sensory clubs.

_fp_ = fibre from subepithelial plexus of subumbrella.

_fph_ = filaments of phacellus.

_frn_ = frenulum.

_ft_ = funnel-shaped depression in ectoderm axial to base of tentacle.

_g_ = gelatine.

_gc_ = ganglion cell.

_ge_ = gelatine of exumbrella.

_go_ = gastric ostium.

_gs_ = gelatine of subumbrella.

_hvl_ = horizontal vascular lamella.

_i_ = interradius.

_if_ = interradial funnel of bell cavity.

_ifr_ = interradial furrow.

_ivl_ = interradial vascular lamella.

_l_ = lens.

_lv_ = lip of valve.

_m_ = bell margin.

_mc_ = mucous cell.

_mep_ = mesogonial pocket.

_mo_ = mouth.

_mp_ = marginal pocket.

_mp´_ = smaller marginal pockets, in Tripedalia.

_mst_ = muscle of stock of sensory club.

_mt_ = muscle at base of tentacle.

_n_ = nerve.

_nc_ = network cells, in sensory club.

_nf_ = nerve fibres.

_nm_ = nematocyst.

_nst_ = nerve of stalk of sensory club.

_osn_ = outline of sensory niche.

_p_ = perradius.

_pe_ = pedalium.

_ph_ = phacellus.

_pr_ = proboscis.

_r_ = reproductive organ.

_rc_ = remains of concretion.

_rcl_ = radial canal.

_rg_ = radial ganglion.

_rm_ = radial muscle.

_rn_ = radial nerve.

_rns_ = root of nerve of sensory club.

_rs?_ = remains of stalk (?) of sensory organ.

_rt_ = retina.

_s_ = stomach.

_sc_ = bell cavity.

_scl_ = sensory club.

_scn_ = supporting cell of nerve.

_se_ = sensory epithelium.

_sh_ = shrinkage space.

_sl_ = stalk of lens.

_sla_ = supporting lamella.

_sn_ = sensory niche.

_so_ = sensory organ in proboscis of Tripedalia.

_sp_ = stomach pocket.

_sph_ = stalk of phacellus.

_ss_ = stalk of sensory organ, in proboscis.

_st_ = stalk of sensory club.

_su_ = suspensorium.

_sub_ = subumbrella.

_tl_ = lateral tentacle.

_tm_ = median tentacle.

_v_ = velarium.

_va_ = vacuole.

_vb_ = vitreous body.

_vc_ = velar canals.

_ve_ = edge of velarium.

_vfs_ = visual fibres, according to Schewiakoff.

_vg_ = valve of gastric ostium.

_vl_ = vascular lamella.

_vlc_ = vascular lamella connecting _vls_ with _vlm_.

_vlm_ = vascular lamella of margin.

_vls_ = vascular lamella of sensory niche.

_vlst_ = vascular lamella of sensory niche at base of stalk.

_wc_ = wandering cells.

_w-x-y-z_ = successive levels of Figs. 40-43 on Fig. 5.

DESCRIPTION OF FIGURES.

Fig. 1. Charybdea Xaymacana, from one of the four interradial sides.

Fig. 2. The same from above.

Fig. 3. The same from below, the four tentacles cut off.

Fig. 4. The same cut in halves vertically (or radially) through a perradius.

Fig. 5. The same out in halves vertically (or radially) through an interradius.

Figs. 6-16. Diagrams of horizontal (or transverse) sections through C. Xaymacana at successive levels.

Fig. 17. Tripedalia cystophora, from one of the four interradial sides.

Fig. 18. The same from below.

Fig. 19. The same cut in halves vertically through a perradius.

Fig. 20. The same cut in halves vertically through interradius.

Figs. 21-30. Diagrams of horizontal sections through T. cystophora at successive levels.

(The following are of Charybdea, except when specially stated otherwise.)

Fig. 31. Horizontal section through the suspensorium.

Fig. 32. Diagram of a gastric ostium seen from the stomach side.

Fig. 33. Diagram of a vertical section through a gastric ostium.

Fig. 34. Diagram of a horizontal section through a gastric ostium.

Fig. 35. Diagram to illustrate the formation of the central and peripheral gastro-vascular systems of a Hydromedusa (_a_, _b_, and _c_) and a Cubomedusa (_d_).

Fig. 36. Vertical section through the upper part of the bell, adradial, to show horizontal vascular lamella.

Fig. 37. Vertical section through the perradius, to show vascular lamella of the niche of the margin.

Fig. 38. Vertical section a little to one side of the last, to show same structure.

Fig. 39. Horizontal section through the upper part of the sensory niche, to show vascular lamella of the niche.

Figs. 40-43. Horizontal sections through the base of a pedalium at successive levels, _w-x-y-z_, Fig. 5, to show marginal lamella.

Fig. 44. Diagram to show relations of sensory niche, of bell margin and velarium in adult Tripedalia. The velarium represented as pendant.

Fig. 45. To show the same structure in a young Tripedalia.

Fig. 46. Horizontal section through the last just at the margin, to compare with Fig. 29.

Fig. 47. Cross-section through the nerve ring.

Fig. 48. The structure of the nerve as seen by focusing at successive levels.

Fig. 49. Diagram to show the relation of the nerve ring to the sensory club.

Fig. 50. Horizontal section through the upper part of the sensory niche, to show passage of nerve root through gelatine of subumbrella to stalk of sensory club.

Fig. 51. Vertical section through base of stalk of sensory club, to show same passage.

Fig. 52. Similar section to last, but nearer to perradius, to show sub-endodermal tract of nerve fibres.

Fig. 53. Sensory organ in proboscis of Tripedalia, as seen from surface in living animal.

Figs. 54 and 55. Sections of same sensory organ.

Fig. 56. Vertical section through one side of proboscis, to show sensory organ attached to endoderm. (Tripedalia.)

Fig. 57. Diagram of the outlines of sensory club seen from the side, by camera lucida.

Fig. 58. Part of retina of larger complex eye cut radially.

Figs. 59-62. Four sections in direct sequence through retinal cells transversely, larger eye.

Fig. 63. Transverse section through the tips of cells of a slightly pigmented retina, larger eye.

Figs. 64-66. Three transverse sections through vitreous body at different levels. All from same series, but not in direct sequence; larger eye.

Fig. 67. Radial section through retina, to show fibres from the long pigment cells; larger eye.

Fig. 68. Transverse section through vitreous body of Tripedalia near retina.

Fig. 69. Vertical section through smaller complex eye.

Fig. 70. Wandering cells, Charybdea.

Fig. 71. Floating mass, from stomach pocket of Tripedalia.

Fig. 72. Horizontal section through larger complex eye. (See text figure, p. 50.)