already been described (Claus, Schewiakoff, Conant), I shall not tarry
long in this respect. Speaking generally, these eyes are flask-shaped
(Fig. 12), the proximal pair quite so, while the distal pair are drawn
out in the transverse diameter of the club. These eyes are invaginations
of the surface epithelium and the shape of the cells lining these
invaginations is quite like that of the epithelial cells, except that
their distal portions (bordering the lumen of the invagination) are
heavily pigmented. The proximal walls (Fig. 12, left side) of the distal
pair are heavier pigmented than the distal walls and the proximal pair
of eyes. Schewiakoff calls attention to this point. The pigmentation is,
furthermore, not only heavier, but the pigmented portion of each cell is
much longer in the proximal walls of the distal eyes (indeed, the cells
are longer) than in the distal walls. The significance of this I do not
understand. Indeed, I am inclined to believe that in life all these eyes
are pigmented quite alike and that it is the reagents used that alter or
dissolve the pigment in certain places. Yet, the fact that the cells
of the proximal walls of the distal eyes have their pigmented portions
nearly double the usual length, shows some deeper significance.
I also note here the small secondary, non-pigmented invagination into
the tissue of the clubs from each of the distal simple eyes. Schewiakoff
describes this invagination, and it extends in a proximal and dorsal
direction (dorsal-side of club opposite complex eye) from the dorsal
sides of the distal simple eyes. The cells of these invaginations are
not pigmented, but quite like the other pigmented cells in shape, and
like these with distal flagellate fibers. I do not see the necessity
of assuming, however, that these secondary invaginations are the real
sensitive parts of these eyes, while the pigmented parts serve as an
iris, as Schewiakoff does in his general discussion.
The histological structure of both pairs of simple eyes is the same.
Sections and macerations give me evidence of only one kind of cells,
all pigmented alike (except, of course, the non-pigmented secondary
invaginations just noted). The cells in these eyes are very closely
crowded so that their nuclei lie at several different levels. That they
all extend to the lumen of the eyes and are all pigmented could be
demonstrated with certainty in many sections, when some of these cells
whose nuclei lay most centrad could be followed with the greatest nicety
to the lumen (Fig. 12). Macerations (Figs. 8, unlettered cells 21) also
show cells with very long cell bodies pigmented at their distal ends and
occasionally with a distal process or fiber. While there are, therefore,
spindle-shaped cells found, yet they are in every other respect alike,
and their differences of shape and position of nuclei are simply the
result of crowding. There is, therefore, no evidence of supporting
(pigmented) cells and spindle-shaped visual cells (pigmented only
externally) as Claus and Schewiakoff have described and which Conant and
myself cannot corroborate.
Distally, the retinal cells of the simple eyes have each a fiber
(flagellum) that extends into the lumen (Figs. 12, 15, 16, 21). Each
flagellum has a dumbbell-shaped basal body just on its entrance into
its cell quite like the basal bodies described for the visual cells of
the complex eyes (Fig. 12, part left unpigmented). Each flagellum, or
fiber, can usually be seen to extend into the cell. In one series I found
appearances like Fig. 16, which is a drawing of a part of a section
through one of the proximal simple eyes. This section is quite in the
angle between the proximal complex eye and the group of network cells in
the upper part of the club. In this series I could very definitely trace
the distal fibers of the retinal cells centrad, past the nucleus and into
the subretinal nerve-tissue. These fibers could be so easily followed
that no doubt can exist as to the fact noted. It thus appears that the
axial fibers just described pass centrad through the cells and are
continued as nerve fibers. On the evidence of such sections as Fig. 16 I
have indicated these fibers as extending centrad through their cells. The
lumen of the simple eyes is filled with a homogeneous vitreous secretion.
This is often incomplete in some parts; occasionally the secretion
shows a formation of globules, but all this I believe to be due to the
action of reagents. Indeed, I have found simple eyes in which hardly any
secretion was present, while others showed an almost completely filled
cavity. In that portion of the vitreous secretion just outside the mouth
of the distal eyes I occasionally found numbers of very darkly staining
granules. I suspect that these are either bacterial or algal organisms.
As already noted, Claus and Schewiakoff describe two kinds of cells
for the retinas of these eyes which neither Conant nor myself can
demonstrate. Further, I believe I have shown that only one kind exists.
If any doubt should still exist, a section like Fig. 25 (which is from
the epithelium of the club, but similar smaller areas with central
dots could often be demonstrated in transverse sections of the retinal
cells of the simple eyes) I believe should be convincing. Schewiakoff
further describes flagella for the retinal cells (his visual cells) of
the simple eyes quite as I have described them for all the cells. The
pigmentation that Schewiakoff mentions as occurring in the secretions
within the lumina of these eyes I believe to have been dissolved in from
the pigmented zones. I find no definite pigmentation in these vitreous
secretions. These secretions are evidently products of the retinal cells
and have been so regarded by former observers.
_Lithocyst and Concretion._--The cavity filled by the concretion is
lined in places by a single layer of cells, two of which are shown in
Fig. 7. This fact has been noted by both H. V. Wilson and Conant. Such
cells are evidently remnants of the cells that formed the concretion. The
supporting lamella completely surrounds the cavity of the concretion.
The concretion filling the lithocyst has the shape of a hemiprolate
spheroid cut in the plane of the axis of revolution. Whether it is of
endo- or of ectodermal origin, I believe developmental studies only can
determine. Tests made in the Chemical Laboratory show the presence of
calcium sulphate with perhaps a very small trace of phosphate.[i] Nitric
acid slowly dissolves these concretions, but I believe Claus was mistaken
when he said that they dissolve with an evolution of gas. I watched
them dissolve under the microscope, and never could see the least bit
of gas formed. If Claus’s observation is correct, then the composition
of the concretions of C. marsupialis is different from that of the
concretions of C. Xaymacana. The concretions, further, were dissolved out
of the material preserved in formaline and in osmic acid solutions. For
dissolving them in situ I used either nitric or hydrochloric acid, or
both. A slight husk remains after all the lime is dissolved.
_The Epithelium of the Clubs._--The epithelium is thickest on the dorsal
side of a club. The thickening here, as in several other places, seems
to be due to a crowding of the cells, in consequence of which the nuclei
come to lie at different levels, but I believe that all the cells quite
reach the surface. The cells with their nuclei nearest the surface are
pyramidal in shape, with the bases of the pyramids toward the surface,
while those cells whose nuclei lie deeper (where several layers of nuclei
occur) may be spindle-shaped (Figs. 12, 23, 24, 26). Centrad these cells
are continued into a single process, which often seems to extend to the
basement membrane (Figs. 7, 12, 13, 23, 24). Where the epithelium covers
the region of the concretion, the cells become flattened and with the
long axis of their nuclei parallel with the surface of the club (Fig. 7).
The same holds true for the corneal epithelium (Figs. 7, 13).
It is a significant fact that in many places the nuclei form only a
single layer, and in such places one cannot speak of spindle-shaped
cells. I cannot find any evidence of sensory and supporting cells as
Schewiakoff describes. The fact that spindle-shaped cells may exist is
simply a physical consequence of their being closely crowded. Conant
arrived at the same conclusion.
But I have another and better reason for supposing the existence of only
one kind of cells in the epithelium. In a tangential section taken just
through the tips of the epithelial cells (Fig. 25) I find polygonal areas
with a central dot. This section does not at all agree with Schewiakoff’s
Fig. 8, in which he figures two kinds of cells. In Fig. 25 there can be
no evidence of two kinds of cells, unless both kinds have like flagella,
for these dots are the transverse sections of flagella continued within
the cells (Fig. 26).
The epithelium, then, is flagellate, a flagellum to a cell. Whether there
are flagella on the epithelium covering the region of the concretion, I
could not determine. But I believe that in all other parts, excepting,
of course, the corneas, it is flagellated. The fibers (flagella) of the
simple eyes are evidently the flagella of the invaginated epithelium.
Each flagellum has a basal body, and I could in many instances determine
that it was dumbbell-shaped (Fig. 12). This fact was not always evident,
however, and it was only occasionally that I felt sure of it. Often
the flagella showed only a general thickening within the cells (Fig.
26) while, again, the thickening (basal body) might be quite localized
near the surface of the cell. Each flagellum extends into its cell,
and occasionally I could trace one clear past the nucleus into the
subepithelial nerve-tissue (Fig. 26), just as I did for the axial fibers
of the retinal cells of the simple eyes. In those instances in which I
could do this, the fibers could so clearly be traced that little if any
doubt can exist. I have thus made bold and have drawn the flagella as
continued through their cells into the subepithelial nerve-tissue for all
the cells of the epithelium of Fig. 12.
A word on the epithelium covering the network cells of Fig. 13. Conant
and Schewiakoff here describe fibers from the supporting lamellæ that
pass in bundles in among the network cells. These fibers are supposed to
be a part of the supporting lamella which reaches out to be a support
for the epithelial cells. (Schewiakoff also describes similar fibers for
other parts of the epithelium.) Now, as Conant himself shows in Fig.
13, these coarse fibers are not of the same consistency and staining
capacity as the supporting lamella. I found them to stain just like the
intracellular parts of the flagella or like the central continuations
of the axial fibers of the cells of the simple eyes. I could, also,
occasionally trace them to the surface of the epithelium, and beyond,
when they became continued as short blunt processes or flagella (Fig.
13). I, therefore, conclude that they are sensory fibers like those I
have described for the other epithelial cells. Yet, that they pass to
the supporting lamella, just as Conant shows in Fig. 13, would seem to
indicate that they are fibers from the supporting lamella or processes of
the epithelial cells. While this stands as an objection to their being
sensory fibers, yet I cannot explain away their being continued distally
as a flagellum, except I assume this continuation to be an artefact.
This does not seem probable. Perhaps they serve both purposes; namely,
that the cell body with its axial fiber is continued to the supporting
lamella, the cell proper ending there, while the axial fiber is continued
as a nerve fiber. I believe this to be the proper explanation.
The epithelium of the peduncle is quite like the epithelium of the
club just described. Sections through the tips of the epithelial cells
of the peduncle and also sections sagittal to the axis of these cells
give sections like Figs. 25 and 26. I, therefore, conclude that this
epithelium is a sensory flagellate epithelium like that of the clubs.
Nerve tissue and unstriped muscle fibers underly the epithelium of the
peduncles. Claus and Conant also describe a small ventral endodermal
tract of nerve tissue, which according to Conant is connected with the
endodermal nerve tissue found in the region of the radial ganglia.
To sum up, the epithelium of the club and the peduncle is a flagellate
sensory epithelium whose flagella are continued through the cells as
nerve fibers into the nerve tissue below. _A priori_, judging from the
mass of nerve tissue underlying the epithelium, we should expect the
epithelium to be one strictly sensory. What sense it serves is difficult
to surmise. In the physiological part of this paper I suggested that it
might be tactile, serving in connection with the lithocysts in giving the
animal sensations of space relations.
Claus mentions having seen patches of flagella on the epithelium of the
clubs. Schewiakoff supposes that his spindle-shaped sensory cells have
only a single flagellum, while his supporting cells have many cilia.
In the latter supposition he was evidently mistaken. Conant (from an
unpublished note) saw the flagella of the epithelium on the living
object and does not think that there could be more than a single one to
each cell. He also concludes from living specimens squeezed out under a
cover-glass, that there is only one kind of cells in the ectoderm.
Cilia and flagella extending into the cells to which they are attached
are described by a number of observers.
I shall not endeavor to discuss the subject further, but shall append the
literature on the subject that has come to my notice. (See Literature).
Some of these observers ascribe a nervous function to these centrad
continuations. I am inclined to believe that they represent the primitive
fibrils of Apathy, whether the cilia or flagella are motile or sensory. I
should mention, however, that Apathy has traced the “Primitivfibrillen”
to be continuous with cilia, and also traces them into the sensory rods
of the sensory cells in the sense organs of leeches. Eimer also describes
cilia as continued centrad.
_The Network Cells and the Multipolar Ganglion Cells._--Conant is the
first to accurately describe the true structure of the network cells
(Fig. 13) that fill the upper part of the club between the proximal
complex eye and the attachment of the peduncle. I cannot add anything
to Conant’s description. As their name implies, they are filled with
a coarse network-like structure with a central nucleus and nucleolus.
Schewiakoff erroneously described them as ganglion cells and Claus as
supporting cells. I have sometimes thought that they are not made up of
a network, but of a vesicular structure; _i. e._ the network we see is
really produced by the sections of planes that intersect to form little
polyhedral cavities. I could not, however, satisfy myself on this point.
I further saw similar but smaller cells, with a finer network, disposed
in small groups laterally and distally from the attachment of the
peduncle to the club.
What the function of these network cells is can only be guessed. In size
and shape they somewhat resemble some of the cells found in luminous
organs. Conant, however, nowhere mentions that Charybdea is luminous.
Lateral to the larger group of network cells lie two groups of large
multipolar ganglion cells (a group on each side). Claus describes these
cells, but Schewiakoff does not specially note them, and evidently
considered them a part of the network cells, which he erroneously
described as ganglion cells.
_The Nerve Tissue._--I cannot add anything new on this. It consists of
fine fibers and ganglion cells, quite as described by Claus, Schewiakoff,
and Conant, and fills the club between the ampulla and the epithelium,
except the spaces occupied by the eyes, lithocyst, and network cells.
It is likewise present under the ectoderm of the peduncle, where also
a small tract is found under the endoderm. (See preceding head, or
Claus[3], and Conant[8b]). As already noted, under the distal complex
eye, I find only large nuclei to represent the ganglion cells. By saying
this, however, I do not wish to dispute their ganglionic nature. The
large multipolar ganglion cells I have noted under the preceding topic.
_The Supporting Lamella._--The supporting lamella is a continuation,
through the peduncle, of the jelly of the bell. It completely surrounds
the ampulla and the lithocyst, and also forms a partition between them,
so that, as already noted, the lithocyst becomes completely surrounded
by it. It also sends a partition ventrally between the complex eyes
(Figs. 7, 13). Its thickening to form a support for the lens of the
proximal complex eye has already been noticed. I shall limit myself in
the discussion of the supporting lamella to the above short resumé, since
Schewiakoff gives further detail.
_The Endothelium of the Ampulla and the “Floating Cells.”_--The ampulla
is lined by a secreting epithelium. This is shown by the large masses
of a secretion within the bases of the cells, and by smaller masses
scattered in the central and more distal parts (Figs. 7, and 27, lower
half). The section of the cells is such in Fig. 7, that the bases of some
(those nearest the supporting lamella) are taken, the central nuclear
region of others, and the tips of those farthest from the supporting
lamella. The section may be said to be taken diagonally through the bases
and central parts of some of the cells, but owing to the curvature of
the ampulla wall, through the tips of others. The secretion is a colloid
substance, staining yellowish gray with iron-hæmatoxylin, blue with Lyons
blue, and reddish with borax-carmine. Sometimes darkly staining rods and
fibers of unknown origin could be seen within the larger masses of the
secretion (Fig. 7). These rods and fibers could also be seen in spaces
within the cells, from which the secretion had evidently been dissolved.
I think there can be no question but that the masses described are a
secretion. Many series, however, do not show it; indeed, an examination
of Conant’s slides gave me little evidence of a secreting function,
though I could demonstrate it in his sections both within the endothelium
and also the floating bodies. The presence or absence of this secretion
is evidently correlated with the feeding habits of the animals, or else
it would be more generally present.
The endothelium is thickest (the cells are longest) in the upper part
of the ampulla where the supporting lamella approaches the lens of the
proximal complex eye, and in the lower portion of the ampulla (Fig. 7),
in the angle between the concretion cavity and the region of the distal
complex eye. In general, the cells are longest in the upper part of
the ampulla, while in the lower part, especially where they cover the
concretion cavity and the dorsal wall, they may be quite cubical instead
of columnar. Often they present a vacuolated appearance at their bases
(Fig. 27). Claus and Schewiakoff describe and figure this endothelium,
but not in detail. No one, to my knowledge, has described this secretory
function.
The nuclei of these cells are peculiar. They may contain a network with
a nucleus (Fig. 27). Again, they may show evidence of amitotic division
(Fig. 20, h, i, j). Indeed, Remak’s scheme (Wilson[18] “The Cell,” p. 46)
can be quite readily demonstrated. It is, however, such dumbbell-shaped,
elliptical, or ringed nuclei as seen in Figs. 7 and 20 that are of
special interest.
I have spoken of some of these nuclei as dumbbell-shaped, elliptical, or
ringed. This is so, however, only in sections. They are really flattened
spheres with a rod of tissue, of the same structure as the nuclear wall,
stretching between the poles. One may conveniently compare the shape of
these nuclei with that of an apple, the core of the apple representing
the rod connecting the two opposite flattened or slightly hollowed poles
of the nucleus. For convenience I shall call the rod connecting the two
poles the axis of the nucleus. The dumbbell or elliptical shape would
be obtained by a meridional section through the axis (Figs. 20, a, b,
c, e, g, k, l, m, n, o, 7). Likewise a ringed appearance with a central
dot would be obtained by a section parallel with the flattened surfaces
or perpendicular to the axis (Figs. 20, d, 7). In a section not strictly
meridional the axis would be cut as in Fig. 29, a, or not show at all.
As nearly as I could determine, the inside of these nuclei is a vacuole,
which the axis penetrates.
The walls and axis of these nuclei have the structure of a very fine and
dense network that stains very dark with iron-hæmatoxylin. It stains
quite like the reticulum of any nucleus, but is very dense, as though
all the reticulum of the nucleus had been crowded together at the
surface. Judging from appearances like p (Fig. 20), the hollowing out, so
to speak, of these nuclei, would seem to be a process of vacuolation, the
reticulum becoming crowded aside to the surface. But how, on this view,
to amount for the formation of the axis, I do not know. Perhaps the axis
is formed by a pushing in of two opposite poles of a nucleus, the two
invaginations meeting and fusing. On this supposition one might expect
the axis to be hollow (cylindrical), but I could not determine that it
was. Perhaps the centrosphere (or spheres) (see the next paragraph) has
something to do with the formation of the axis (Fig. 20, b, g, e, etc.).
In the nuclei of Fig. 20 with the dark outlines, and of Fig. 7 a small
reticular body is seen just opposite one end of the axis, or opposite
both ends in g. In d (Fig. 20) this body is seen next the axis just
below (outside) the hollow cup represented by the hollow ring. In this
instance a central granule is seen in the reticular body, as also in
c. I take this reticular body to be the centrosphere, and the central
granule in c and d the centrosome. In k, l, m, n, and o (Fig. 20), which
are from another series, in which the walls of the nuclei did not stain
so dark as in the other nuclei of the same figure, a nucleolus could be
definitely seen, indeed, sometimes quite perched upon the wall of the
nucleus (k, l). In several instances I could see two nuclei, as in o. But
besides these nucleoli, I could in several instances see quite definitely
a reticular body (centrosphere) opposite the axis (m, n, o) quite as I
described for the nuclei with the dark outlines. In a, b, c, d, e and g
the nuclei could not be so readily demonstrated, but I could occasionally
see a darker stained body as in a, c and g, that I have no doubt is the
nucleolus, which here, again, is perched quite upon the surface of the
nucleus. This position of the nucleolus is perhaps due to its having been
crowded to one side by the nucleus becoming hollow. It is no uncommon
thing, either, to find several nuclei in a single cell, sometimes in
process of division or just divided as o and e (Fig. 20), also h, i and
j. The whole nuclear phenomenon that I have described seems to be one of
division. Perhaps it is somehow associated with the giving off of the
secretion of the cells, for these nuclei seem to be found in greatest
abundance in those cells in which the secretion is most abundant. In
Conant’s sections I found but little evidence of these nuclear phenomena
as also little secretion, which all goes to show the association of
the nuclear phenomenon with the secretion. I have failed to find any
descriptions in the literature of nuclei to which I could refer my
observations.