A: THE VASCULAR LAMELLÆ.
In Medusæ it is a common thing to find that in certain definite places of
the gastro-vascular system two endodermal surfaces that were primarily
separated by a space have come together and fused into a single lamella
or plate. Such a structure is called indifferently a cathammal plate,
an endodermal lamella, or a vascular lamella. In the adult animal the
vascular lamellæ are by virtue of their very nature formations “with a
past.” They are scaffolding left in the completed structure, giving us
clues as to the way in which that structure was brought about; and in the
Cubomedusæ, whose development is as yet unknown, they therefore afford an
unusually interesting subject for special consideration.
The vascular lamellæ that are found in Charybdea and Tripedalia may for
convenience be described as forming two systems, the internal and the
marginal. The former comprises the endodermal fusions that separate the
stomach from the stomach pockets (except for the spaces of communication
left free, the gastric ostia) and those that separate the stomach pockets
from one another. The marginal system consists of the lamella that
connects _endoderm_ of the gastro-vascular system with _ectoderm_ of
the surface in a ring all around the bell margin, and with it also the
vascular lamella of the sensory niche, which has already been referred
to in the general description of Charybdea. The lamellæ of the internal
system have been described by previous writers, and especially by Claus
in his paper on Charybdea, but they are still in need of comprehensive
and clear treatment. The lamellæ of the margin and of the sensory niche
have also been described by Claus, but not thoroughly or with entire
accuracy, nor did he recognize the vascular lamellæ of the sensory niche
as originally a part of the lamellæ of the margin. This last was first
determined by H. V. Wilson upon specimens of Chiropsalmus quadrumanus
obtained at Beaufort, North Carolina. Professor Wilson’s unpublished
notes on Chiropsalmus were very kindly placed in my hands, and so far
as the vascular lamellæ are concerned my own work is only a confirmation
and amplification of his, since Charybdea and Tripedalia in this respect
agree with Chiropsalmus.
The vascular lamellæ of the internal system are the most prominent and
morphologically the most important. They comprise the four vertical
strips of fusion that separate the four stomach pockets in the interradii
(_ivl_ in the figures of the series of cross-sections of Charybdea and
Tripedalia, Nos. 6-15 and 21-29), and four curved horizontal cross-pieces
at the top of these which separate the stomach from the stomach pockets,
and would make the separation complete did they not leave in each
perradius a free space between their ends, which makes possible the
gastric ostia.
The arrangement of this internal system of vascular lamellæ is simple.
What they amount to is a certain definite number of linear adhesions
between the two walls of an originally undivided gastro-vascular space,
by which that space is divided up into a central stomach and a peripheral
portion, and the peripheral portion thus further divided into the four
stomach pockets. Perhaps the idea may be conveyed by likening the whole
medusa to a couple of bowls fitting closely one within another and
plastered together at the margins. The exumbrella then would correspond
to the outer bowl, the subumbrella to the smaller inner bowl, and the
original undivided gastro-vascular space to the space between the two.
If now the walls of the space be cemented together in four horizontal
curved lines just in the plane where the bottoms are bending round to
become the sides of the bowls, leaving four interspaces between the ends
of the lines, we should have the original space divided into a central
horizontal somewhat lens-shaped region between the bottoms of the two
bowls that would correspond to the central stomach, and a peripheral
vertical portion between the sides of the bowls that would correspond to
the peripheral gastro-vascular system; central and peripheral portions
would communicate by the four interspaces between the lines of fusion,
which would correspond to the four gastric ostia. If, further, the
vertical peripheral portion be subdivided by four more lines of fusion
running vertically at equal distances apart, each connecting above
with the middle point of the corresponding horizontal line of fusion,
we should have the simple peripheral portion divided into four parts,
corresponding to the stomach pockets, by four vertical lines of fusion,
corresponding to the four interradial vascular lamellæ, the _ivl_ of the
figures.
These mutual relations of stomach, stomach pockets and lamellæ will
perhaps be made clearer if a comparison is drawn between them and the
similar structures of a Hydromedusa. Liriope, one of the Trachomedusæ, is
a good form to take for such a comparison, since by reason of its direct
development from the egg it is free from the complications of hydroid
medusæ. The young medusa has at first a simple, undivided gastro-vascular
cavity which later is divided up into the central stomach and the
typical radial to circular canals of the Hydromedusæ by means of fusions
between the two endodermal surfaces. Diagrams _a_, _b_ and _c_ of Fig.
35 represent very schematically this process of division into stomach
and canals. In _a_ we have a projection upon a plane surface of the
primary, undivided gastro-vascular cavity, as seen from above; _b_ shows
the first four points of fusion in the interradii; _c_ represents those
four points expanded by growth in all directions into broad cathammal
plates in such a way as to leave the stomach in the centre, the radial
canals in the perradii, and the circular canal in the periphery as all
that remains open of the primary simple cavity. These broad plates
of vascular lamella, separating the narrow radial canals, persist in
the adult Liriope to tell the tale of the formation of the definitive
gastro-vascular system. It seems to me that we are justified by analogy
in drawing a similar conclusion for the Cubomedusæ. In _d_ of Fig. 35 is
represented a projection of a Cubomedusa, in which the homology of the
stomach pockets with the radial canals of the Hydromedusa, and of the
narrow strips of fusion with the broad cathammal plates, is shown at a
glance. To make the comparison more perfect we have only to remember that
in the Cubomedusæ there exists below each interradial vascular lamella a
connecting canal (Figs. 16, 29 and 35 _d_, _cc_) uniting the two separate
adjacent pockets. This, as has been pointed out by other writers, is the
representative of the circular canal of the Hydromedusæ. Practically the
only difference between the structure of the gastro-vascular system of
the Cubomedusæ and that of a form such as Liriope, is that in the latter
the fused areas have broadened out at the expense of the radial canals,
while in the Cubomedusæ on the contrary they have become long and narrow.
One is strongly tempted by the foregoing comparison to speculate a little
as to whether the reproductive organs of the Cubomedusæ, which lie _in_
the stomach pockets and are generally supposed to be endodermal, may not
bear some closer relation to those of the Trachomedusæ, which lie “in the
course of” the radial canals (Lang’s Text-book) and by common consent
are ectodermal. And while we are being led by facts such as those just
mentioned above to wonder just a little whether after all the position
of the Cubomedusæ among the Acraspeda is so firmly assured--doubting
some, yet in the frame of mind of one who “fears a doubt as wrong”--the
velarium suggests itself as another point in question. Haeckel does not
hesitate to state emphatically that the velarium of the Cubomedusæ and
the velum of the Craspedote medusæ are only analogous, but the reasons
that he gives (sie sind unabhängig von einander entstanden, und ihre
Structur ist zwar ähnlich, aber keineswegs identisch; namentlich das
Verhalten zum Nervenring ist wesentlich verschieden: System, p. 426)
somehow do not produce so much impression upon one as the very velum-like
appearance of the velarium itself. The origin from the fusion of marginal
lobes is not as yet a matter of observation, and the relation to the
nerve ring is not essentially different from that of the velum to the
lower (_i. e._ inner) nerve ring in the Craspedotæ. The four frenula and
the diverticula from the gastro-vascular system seem to be the chief
differences in structure after all, and these Haeckel evidently did not
think worth mentioning. This speculation, as to the possible relation of
the Cubomedusæ to such forms of the veiled medusæ as Liriope, though it
may be very tempting, is scarcely fruitful enough to repay much effort
on the part of either reader or writer. The whole subject must remain
uncertain until the facts of the development of the Cubomedusæ are known.
If the structure of the vascular lamellæ of the internal system has been
made clear, the appearances of the vertical and horizontal components in
the figures will be understood without much further explanation. The four
vertical strips in the interradii (_ivl_) have been already referred to
in the figures of the cross-sections of both Charybdea and Tripedalia. In
the longitudinal sections of the two jelly-fish through the interradii,
the vertical lamellæ are cut throughout their entire length from stomach
to connecting canals (Figs. 5-20, _ivl_). The horizontal cross-pieces at
the tops of the vertical lamellæ also appear in several of the figures.
Fig. 36 represents the appearance that would be given by a longitudinal
section taken through any portion of the upper part of the bell except
in the interradii, or in the perradii, through the gastric ostia. The
horizontal vascular lamella (_hvl_) is shown connecting the endoderm
of the stomach (_ens_) with that of the stomach pocket (_enp_). In a
longitudinal section directly through an interradius (Fig. 5 or 20) the
horizontal lamella is cut just at the point where it joins the vertical,
so that the two are not differentiated. In a section through the region
of a perradius (Fig. 4 or 19) the horizontal lamella is of course not
cut, since the section passes through the gastric ostium, whose existence
is conditional upon fusion not having taken place between the endodermal
surfaces.
The first figure in each of the series of cross-sections (Figs. 6 and 21)
also shows the horizontal vascular lamella, cut across slantingly twice
in each quadrant as it passes between the gelatine of the ex- and of the
subumbrella to connect the epithelium of the stomach with that of the
stomach pocket. The fact that more of the lamella does not appear in such
a cross-section only shows that its course is not perfectly horizontal.
The region in which the same lamella lies is indicated in the surface
view of the top of the bell of Charybdea (Fig. 2) by the bent line _hvl_
in each quadrant. The figure manifests the appropriateness of Claus’s
name for the horizontal lamella--“bogenförmige Verwachsungs-Streifen.”
Haeckel calls the same structures “Pylorus-Klappen,” and in his account
of Charybdea Murrayana in the Challenger Report, speaking of the three
divisions of the stomach (buccal, central and basal) which he traces
upwards from the stalked forms of Scyphomedusæ, he says: “The central
stomach in this Charybdea, as in most Charybdea, is joined to the basal
stomach, as the pyloric stricture between the two is not developed and
only faintly indicated by the slightly projecting pyloric valves.”
Again, in speaking of the valves of the gastric ostia, he says: “These
four perradial ‘pouch valves’ alternate with the interradial pyloric
valves.” It is difficult to understand, however, how the “bogenförmige
Verwachsungs-Streifen” of Claus, which are undoubtedly the same
structures as those which I have called the horizontal lamellæ, and are
only strips of endodermal fusion, can be “projecting pyloric valves,” or
indeed can properly be spoken of as valves at all. Possibly Haeckel was
not quite able to understand Claus’s description, and in his desire to
find something in the stomach of Charybdea which would serve to set off a
central from a basal part, such as is found in the Lucernaridæ, hit upon
Claus’s “Verwachsungs-Streifen.” I have elsewhere given it as my opinion
that in such of the Cubomedusæ as I have studied there is no structure in
evidence that would properly serve to mark a limit between a basal and a
central portion of the stomach.
We have next to describe the marginal system. The vascular lamellæ
mentioned above in every case connected endoderm of one cavity with
endoderm of another; those of the margin have the noteworthy difference
that they run from endoderms of some part of the gastro-vascular system
to _ectoderm of the surface_. The outermost cells of the endodermal
lamellæ make direct connection with the ectodermal cells, without the
usual intervention of a layer of gelatine.
The marginal lamella of Charybdea lies, as the name implies, just on the
bell margin where the edge is curving round into the velarium. All around
the whole circumference of the bell it is found (in Charybdea) at this
same horizontal bend, except in the eight principal radii, where the
tentacles and the sensory clubs have brought about modifications. In any
place except these a vertical section through the margin will show the
marginal lamella connecting the endoderm of the marginal pocket with the
ectoderms of the surface, as represented by _vlm_ in Fig. 38, which is a
vertical section through the sensory niche a little to one side of the
perradial axis.
In the interradii the marginal lamella undergoes modifications due to the
fact that the bases of the pedalia are situated a little upwards from
the exact margin, and that the lamella follows the outline of the bases.
Fig. 1 shows one of the interradial corners of the bell margin looked at
directly from the surface, so that the curved outline of the junction of
the base of the pedalium with the exumbrella is seen. The trace made by
the lamella where it meets the surface ectoderm follows this outline. The
lamella is also shown in the vertical section through the interradius
(Fig. 5 or 20, _vlm_), where it is seen running from the connecting
canals (_cc_), which joins the two adjacent marginal pockets, upwards and
outwards to meet the surface ectoderm. Its course from canal to surface
is not in a direct line, but curved with the concavity upwards. Hence, in
cross-sections at certain levels through the interradial corner it is met
more than once and gives rise to appearances that seem at first sight too
complicated for it to be just the same structure as the simple marginal
lamella described above. That it is the same, and that the complication
is only due to the insertion of the pedalia above the margin, can be
determined by following through a series of cross-sections, the essential
ones of which, as I hope, are given in Figs. 40-43. The levels of these
are shown on Fig. 5 by the letters _w_, _x_, _y_ and _z_, respectively.
Fig. 40 shows the lamella cut but once, just below its highest part. The
section is above the level of the connecting canal and hence still shows
the vertical interradial lamella _ivl_. Fig. 41, at the next lower level
(_x_), shows the same portion of the lamella intersected a little nearer
the interior, while the junction with the endoderm of the connecting
canal is shown still further inside. Fig. 42 is at level _y_, just
through the bend of the loop, so that in part of its course the lamella
is cut almost horizontally, _i. e._ in its own plane. Fig. 43 finally
shows the lamella as it appears below the level of the connecting canal,
cut twice, each portion joining endoderm of marginal pocket with ectoderm
of surface. It thus bears exactly the same relations that it had when we
first met it in Fig. 38 (_vlm_), except that here in Fig. 43 one finds
that a cross-section cuts it at right angles instead of a vertical as in
Fig. 38, as a result of its being pushed upwards from its former position
on the margin by the insertion of the pedalium above the margin.
The vascular lamella of the sensory niche has already been alluded to as
part of the marginal system, and brief reference has been made to it in
the section on the sensory clubs. Like the rest of the marginal lamella,
it connects endoderm with ectoderm. The line that its fusion with the
ectoderm traces on the surface frames in a shield-shaped area at the
bottom of the sensory niche, which is seen in the drawing of the outlines
of the niche, Fig. 44 (_vls_). This lamella was observed by Claus, and
was figured by him both in surface view and in cross-section through the
niche. Apparently, however, he omitted vertical sections through the
niche, so that he supposed that the outline traced by the lamella was
not continuous above, _i. e._ over the stalk of the sensory club (’78,
Fig. 41; text, p. 28). That the outline is closed above, though masked
in surface view by the roof of the sensory niche, is seen at once in
vertical sections, such as Figs. 37 and 38, one of which is directly
through the perradius, the other a little to one side. Both show the
vascular lamella of the sensory niche (_vls_) intersected twice, above
and below the sensory club, and completely cutting off the exumbrella
from any share in the bottom (or inner wall) of the sensory niche. Fig.
39, which is a cross-section through the upper part of the niche, and is
essentially like the similar figure of Claus, shows in like manner that
the bottom of the sensory niche belongs to the subumbrella. H. V. Wilson
was the first to point out, in his unpublished notes, that the lamella of
the niche is complete all round.
In the adult structure of Charybdea and Tripedalia the lamella of
the niche is connected with that of the margin by a vertical strip
of endodermal fusion that does not come to the surface like the rest
of the marginal system, but remains just internal to the gelatine of
the exumbrella, connecting the two adjacent marginal pockets. In the
cross-sections of Charybdea it is seen in Fig. 16 (_vlc_); in those of
Tripedalia it is seen in Figs. 28 and 29. In vertical section it is found
in Figs. 4, 19 and 37. In Fig. 44, which represents the bell margin
and velarium of Tripedalia arranged as if the velarium were vertical
and pendant from the margin (instead of suspended by the frenulum so as
to be at right angles to the vertical plane), the connecting lamella is
shown as a dotted line (_vlc_)--dotted because it does not come to the
surface--joining the lamella of the niche with that of the margin (_vlm_).
The same figure (No. 44) shows a characteristic difference between the
marginal lamella of Tripedalia and that of Charybdea. While in Charybdea,
as Claus points out, the marginal lamella keeps at one level, just a
little above the bell margin, all the way round (except where disturbed
by the special modifications of the tentacles and the sensory clubs),
and never descends into the velarium, in Tripedalia on the other hand
it describes a sinuous course, following the outlines of the marginal
pockets, as is indicated in the figure by the light parallel line _vlm_.
The course as it would be seen in a surface view is obscured just at
each side of the interradius by the overhanging of the bases of the two
lateral pedalia. This is why the lamella is not indicated at these points
in the diagram. The course is seen to lie almost wholly on the velarium,
that is, in the figure below the line which represents the bell margin
proper, the line at which the angle comes when the velarium is in its
normal position, horizontal to the vertical side of the bell.
In this sinuous course of the marginal lamella we have another point of
resemblance between Tripedalia and the Chirodropidæ. H. V. Wilson worked
it out in his sections of Chiropsalmus, and the reconstruction which I
have given in the figure under discussion is in all essentials similar
to his for Chiropsalmus. The differences lie only in the fact that
Chiropsalmus has more velar canals, and that the chief marginal pocket
in each quadrant is not forked peripherally, as is that of Tripedalia
(_mp_), but presents its distal margin parallel to the edge of the
velarium. The two smaller marginal pockets in the perradii (_mp´_) are on
identically the same plan in both.
Tripedalia, having three tentacles joining the umbrella in each
interradius, shows a disturbance of the course of the marginal lamella in
these regions by just so much the more complicated than in Charybdea. The
plan, however, is exactly the same. The lamella is pushed upwards from
the margin by each of the bases of the three pedalia just as is done by
the base of the single pedalium of Charybdea. Fig. 29 shows the lamella
in the same relation to the canal of the central tentacle (_ct_) that
it has in the similar sections of Charybdea (Figs. 16 and 43); and in
addition the first appearances (as the series is traced downwards) of
the arches of the lamella over the two lateral tentacles (_ct´_), which
are inserted a little lower down than the middle one of the group. As
concerns these lateral tentacles, the relations of the vascular lamella
at this level are the same as that in the level of Fig. 40 for Charybdea.
It has been stated more than once already that the vascular lamella of
the sensory niche is a part of the lamella that runs round the margin,
and so far the only evidence given has been the strip of endodermal
fusion running from the marginal lamella to that of the niche. This
strip, however, as has been described, does not come to the surface and
consequently seems at first sight to be a different structure from the
lamella of the margin. That it is not, however, I found very prettily
shown in a series of sections of one of my youngest Tripedalia. In
this the lamella of the niche as it was traced in successive sections
downwards, was found not to form a closed ring at the bottom of the
niche, but each side was continued directly and separately downwards to
the margin, where it passed into the corresponding part of the marginal
lamella. A reconstruction of the condition, similar to that of Fig. 44,
is given in Fig. 45, and I think explains itself at a glance. Evidently
the vascular lamellæ that connect the lamella of the sensory niche with
that of the margin at first come to the surface, like the rest of the
marginal system, but as the animal grows older come to lie within the
gelatine. In this way the condition found in cross-sections just through
the margin of my very small Tripedalia, and represented in Fig. 46,
becomes that of the adult seen in the corresponding portion of Fig. 29.
It is as complete a demonstration as could be required that the lamella
of the sensory niche is at first only a loop of the marginal lamella, a
conclusion that had been already reached by H. V. Wilson on theoretical
considerations, based upon the facts of the adult structure as he found
them in Chiropsalmus.
As Wilson pointed out in his notes, these facts have a close bearing upon
the question of the origin of the velarium. Sixteen marginal pockets
are found in both Chiropsalmus and Tripedalia, and all of them extend
into the velarium. It is not unnatural to suppose that these belong to
sixteen marginal lobes, and that these lobes have fused together to form
the velarium. In the Chirodropus figured by Haeckel (Taf. XXVI) in his
“System” gelatinous lobe-like thickenings are shown in the velarium,
corresponding to the sixteen marginal pockets. In Tripedalia no special
gelatinous thickenings are found, but the arrangement of the marginal
pockets is the same as that of the Chirodropidæ, and perhaps I ought,
when treating of the systematic relations of Tripedalia (p. 5, Fam. III),
to have recognized the analogy to the extent of saying that marginal
lobes may not be completely absent from the velarium of Tripedalia. At
any rate the gelatinous lobes in the case of Chirodropus on the one hand,
and on the other hand the sinuous outline of the margin still mapped out
by the lamella in Chirodropus, Chiropsalmus and Tripedalia, are certainly
very suggestive of an ancestral Cubomedusa in which there was no
velarium, but sixteen free marginal lobes instead. Two more indications
favor slightly the same view. In both Charybdea and Tripedalia a small
notch is seen in the edge of the velarium in the perradius (Fig. 44). Its
constancy suggests that it may not be a chance or meaningless feature.
The second point is the small size of the two marginal pockets adjoining
the perradius. These are in the position of the ephyra lobes of the
Discomedusæ, which always lie on either side of each sensory club, and
which do not keep pace with the other marginal lobes in development. In
the Rhizostome jelly-fish especially they are found much smaller than the
other lobes, as will be seen by a glance at such figures as Haeckel’s for
Lychnorhiza (System, Taf. XXXIV Fig. 2), or for Archirhiza (Taf. XXXVI,
Fig. 5), or Hesse’s figure of the margin of Rhizostoma Cuvieri (’95,
Taf. XXII, Fig. 22). The resemblance between such margins and that of
Tripedalia (Fig. 44), with its simple, unbranched velar canals, is very
suggestive. On the other hand it must be remembered that in considering
the vascular lamellæ of the internal system we found the indication
pointing rather more to Hydromedusan affinities than to any other.
Charybdea throws no light on the question, since it has no marginal lobes
on the velarium and the marginal pockets end strictly at the margin, so
that the only diverticula of the gastro-vascular system in the velarium
are the velar canals.
Before leaving the subject of marginal lobes and pockets I must answer
a possible objection that may occur to some careful reader. It may seem
that I am wrong in holding that there are two marginal pockets in each
octant instead of three, that just as there is one velar canal from
each of the smaller perradial pockets (_mp´_, Fig. 44), so each prong
of the forked larger pocket (_mp_), since it is continued into a velar
canal, ought to be called a marginal pocket likewise, the whole number
of marginal pockets then being twenty-four instead of sixteen. Such a
revision of the terminology would not be without some reason in its
favor, and perhaps a study of more forms would show it to be correct.
But for the present, at any rate, it seemed to me best to abide by the
analogy of Chiropsalmus, in which the peripheral edge of the larger
marginal pocket in each octant is not bow-shaped, but runs parallel to
the edge of the velarium. A revision of the terminology of the marginal
pockets such as implied in the suggestion above would also give rise to
complications when applied to Charybdea, since the latter has no marginal
pockets in the velarium.
As to the functions of the vascular lamellæ, there is too little known to
say much. It is rather improbable that structures retained so definitely
should be mere scaffolding left over from a previous stage of usefulness.
Claus has found in Chrysaora that the lamellæ form a kind of capillary
network in communication with the gastro-vascular system, and he with
others supports the view that they perform an accessory function in
the nutrition of the tissues they penetrate. Upon this point I have no
observations of my own to add.
The marginal vascular lamella is regarded by Claus as perhaps the
vestige of a circular canal around the bell margin. On this subject,
too, I have nothing to add. A lamella of endoderm that connects directly
with the ectoderm of the surface along its whole course is a structure
whose meaning I am wholly unable to understand or even to guess at. A
similar lamella is described by Hesse (’95, p. 430) as occurring in the
ephyra lobes of his Rhizostoma, and he mentions Eimer as the first to
discover this structure, probably meaning the first to discover it in the
Discomedusæ. Whether the lamella is found all around the margin is not
stated. Hesse refers it to the ephyra, and remarks that the investigation
of it in the ephyra would undoubtedly give interesting results.
I will close this part upon the vascular lamellæ with a very pertinent
suggestion made by Professor Brooks to the effect that the usual way
of speaking of the sensory clubs as having moved up from the margin is
looking at the matter in the wrong way. The level of the sensory clubs
undoubtedly represents the original margin, which elsewhere has grown
down and away from its former level, leaving the sensory clubs like
floatage stranded at high-tide mark. Only in this way can the lamella of
the sensory niche have any meaning.
B: THE NERVOUS SYSTEM.
The nervous system of the Cubomedusæ is the most highly developed that is
found in any of the jelly-fishes. If the position of the group among the
Acraspeda is established, it alone is ample to prove that the Hertwigs
had not sufficient evidence when they stated in their monograph on the
nervous system of the Medusæ (’78) that the Acraspeda show a much lower
nervous organization than the Craspedota.
The system naturally groups itself under three heads, the nerve ring, the
sensory clubs, and the motor plexus of fibres and ganglia that underlies
the epithelium of the subumbrella. The general relations of the nerve
ring and of the sensory clubs have been given before in the description
of Charybdea Xaymacana, so that we may pass at once to the consideration
of the finer details of the nervous tissues.
In the structure of the nerve ring I have found myself unable to come
to the same results as those given by Claus, who so far as I know is
the only one that has studied the nerve with special reference to its
histology. Our difference amounts to this, that he finds two distinct
types of cells in the epithelium of the nerve, sensory and supporting,
which would make it a receiving as well as transmitting organ, while I
have not been able to demonstrate satisfactorily the sensory cells, and,
therefore, so far as my own observation is concerned, I am disposed to
attribute to the nerve simply the function of conducting impulses. I do
not know just how much weight to assign to my inability to find evidence
in my sections of the sensory type of cells. Eimer (mentioned by Hesse,
’95, p. 420), the Hertwigs (’78) and Claus (’78) have independently
discovered the two types in one medusa or another, and the Hertwigs,
at least, have demonstrated them by macerated preparations. So far as
Charybdea is concerned, however, Claus had only preserved material and
had to rely upon sections, as have I, since the material which I had
preserved with especial reference to maceration did not turn out well.
The results that we get from sections vary enough for me to believe
that Claus interpreted his sections very much by analogy with other
forms--as indeed, is suggested by his own words (’78, p. 22): “Da es
mir nicht geglückt ist die durch die längere Conservirung in Weingeist
fest vereinigten Elemente zu isoliren, habe ich das muthmassliche
Verhältniss beider Elemente nach Analogie der mir für die Acalephen
bekannt gewordenen Verhältnisse, welche O. und R. Hertwig so schön auch
am Nervenring der Carmarina zur Darstellung gebracht haben, zu ergänzen
versucht.” There can be no doubt of our having the same structures to
deal with, for C. Xaymacana is so much like C. marsupialis as to be
perhaps more worthy of being called a variety of the latter than a
distinct species.
The structure of the nerve as I conceive it is given in Figs. 47 and 48.
The former represents a cross-section, and shows, as others have pointed
out, that the layer of circular muscle fibres (_cm_) is interrupted
by the nerve. It is evident that the tissues which elsewhere on the
subumbrella were differentiated into muscle epithelium and muscle fibre
have here become nerve epithelium and nerve fibre, a point that has not
been remarked upon before, so far as I remember, and that may be of
interest in connection with the neuro-muscular theory. The epithelium
of the nerve (_scn_) is seen to be made up of cells whose inner ends
narrow down into a kind of stalk or process that runs to the gelatine
of the supporting lamella (_gs_) and there joins a little cone of the
gelatine that juts out to meet it. The cells are smaller in general
than those that overlie the muscle layer, especially on the two lateral
margins of the nerve, where they are more crowded together and overarch
the nerve-fibres. The fibres are seen in cross-section between the
processes of the cells. They apparently must lie imbedded in some
clear, watery fluid that does not show in the preserved material. The
processes of the epithelial cells give the fibres the appearance of lying
in alveoli, or being divided into strands, and one of these strands
(_ax_) is always discernible among the others by reason of its more
numerous or finer or more compactly massed fibres. This is the “axis” of
Claus. Here and there in its course appear ganglion cells having their
long axis in the longitudinal direction of the nerve. Elsewhere, in
the nerve as well, and usually nearer to the surface, are found other
ganglion cells, mostly bipolar, some multipolar, which are readily
distinguishable from those of the axis by the fact that their long axis
lies across the nerve. One of these cells is shown in the figure (_gc_).
Here and there in the epithelium alongside the nerve are found mucous
cells (_mc_), distinguished by their clear contents and by the small
exhausted-appearing nucleus at the base with a few threads of protoplasm.
In Fig. 48 I have tried to represent the structure of the nerve by means
of a series of five different views such as would be given by focusing at
five successive levels. In the first (1) we have the epithelium of the
nerve (_scn_ in Fig. 47) in surface view, the cells appearing polygonal
in outline, with here and there a mucous cell. In (2) we find a very
slight layer of ganglion cells and fibres having a transverse direction
(_gc_ and _fp_ in Fig. 47). These are continuous with the plexus of
fibres and ganglion cells which lie above the muscle layer all over the
subumbrella, and which represent the motor part of the nervous system.
This connection with the nerve shows how co-ordination is effected. At
the same level are found fibres of the axis also having a longitudinal
direction. In (3) is seen the main body of fibres, divided in the osmic
preparation from which the drawing was made into irregular wavy strands
which are in all probability largely the result of preservation, but are
in part also due to the separation by processes of the epithelial cells,
as was seen in Fig. 47. The axis is seen with one of its longitudinally
directed bipolar ganglion cells; and at the sides the fibres of the
circular muscle of the subumbrella. These show a slanting direction to
the nerve, due to the fact that the nerve, as mentioned before, has a
sinuous course from the margin in interradius to the level of sensory
club in perradius. At the next focus (4) we come to the gelatine of the
subumbrella (_gs_ in Fig. 47), and below this (5) to the larger polygonal
outlines of the endodermal cells of the stomach pocket (_enp_, Fig. 47),
which like the ectoderm show mucous cells at irregular intervals.
A comparison, now, with Claus’s figures (’78, Taf. II, Figs. 19-21)
will show that, except for the rather unimportant matter of the mucous
cells, which he finds regularly and thickly disposed on each side of
the nerve (’78, Fig. 21), our only essential difference lies in the
matter of sensory cells in the epithelium. His figures show a multitude
of spindle-shaped sensory cells whose central ends are continued in
processes that bend around into the mass of fibres of the nerve. In
his Fig. 20 a relatively small number of nuclei, just one-third as
many, are seen attached nearer to the surface, which represent the
supporting cells. The plan of structure (as shown in his Fig. 20) is an
alternation of (1) supporting cells offering a broad peripheral end to
the surface and having the central end continued as a supporting fibre
to the gelatinous lamella, and (2) spindle-shaped sensory cells with
nuclei at a lower level, which send their peripheral process up between
the supporting cells to the surface, while the central process becomes
continuous with the nerve fibres, often branching into two processes. In
my sections I have not been able to see either a regular alternation of
nuclei at different levels, or central processes which unmistakably bend
round into the nerve fibres. In every case in which I could trace the
central process of a cell clearly it ran to the supporting lamella, and
this whether the nucleus of the cell lay near the surface of the nerve or
deeper down, as in the somewhat spindle-shaped cell seen on the left of
the centre of the nerve in Fig. 47. Of course in many cases the central
process could not be traced in a section, and this leaves room for the
supposition that such were always the sensory cells. From my inability to
demonstrate sensory cells in the nerves of Charybdea, I by no means wish
to deny their existence; for that remains to be proved, or disproved,
by macerations. At any rate, they cannot be so numerous as has been
supposed. The position of the nuclei shows that.
The epithelium of the nerve is said by Claus to be ciliated. It has been
suggested by Schewiakoff that probably in such cases the sensory cells
bear one long cilium, while the supporting cells have many smaller cilia.
Unfortunately, I made no observations upon the ciliation of the nervous
structures of the living animal, and the traces of cilia that are shown
in preparations of preserved material are a poor basis to speculate much
on. Claus considers the sensory cells of the epithelium of the nerve a
special seat of tactile sensation.
The way in which the nerve reaches the sensory clubs is interesting.
Under the topic of the vascular lamellæ it was explained that the sensory
clubs and the bottom of the sensory niche from which they spring are
parts of the subumbrella. Fig. 37 reminds at a glance better than any
other one drawing how the bottom or inner wall of the niche is completely
cut off from the exumbrella by vascular lamellæ above and below the
stalk of the club. From this figure, now, it will readily be understood
that the nerve in order to pass to the base of the stalk has simply
to traverse the gelatine of the subumbrella. This fact, which seems
surprising enough at first sight in view of the position of the clubs
on the external surface of the umbrella, was correctly pointed out and
explained by Claus, but one or two figures will serve perhaps to give a
clearer idea of it.
Fig. 49 is a diagram of the nervous structures in the region of the
sensory niche, as they would be seen on the surface of the subumbrella
turned toward the bell cavity. The outline of the sensory niche as it is
seen through the tissue of the animal is represented by the line _osn_.
The sensory club (_scl_), and its stalk with a conical basal portion are
given by the lightly dotted outline and are also imagined as seen through
the animal. The nerve (_n_), being on the surface of the subumbrella, is
shown as a heavy line describing an arch over the outline of the niche.
In the middle point of the arch is a slight thickening of the nervous
tissue (_rg_) which shows in section a large increase in the number of
ganglion cells, and is the radial ganglion of Claus. The same is seen,
exaggerated in size, in Fig. 12. From it there extends upward a slender
strand of nervous tissue (_rn_), the radial nerve of Claus. In Charybdea
this can be traced but a very short distance. In Tripedalia it is much
more distinct and traceable for a longer distance, and I might say in
passing that this and the sensory organs in the proboscis are the only
differences I have noted between the nervous systems of Tripedalia and
Charybdea.
Nerve ring, radial ganglion and radial nerve all lie on the bell cavity
surface of the subumbrella. The way, now, in which the nerve ring reaches
the base of the stalk is simply by sending two roots through the gelatine
of the subumbrella to the conical base of the stalk. These roots are seen
in the diagram at _rns_. After passing through the gelatine the roots
come together on the inner side of the base--that is, the side turned
toward the bell cavity--and then pass downwards (_nst_) on the inner side
of the stalk of the club to the mass of nervous tissue at its end.
This passage of nervous tissue through the gelatine in order to reach the
sensory club is a little hard to grasp at the first, and I have tried
to render it more intelligible by a couple of drawings of sections.
Fig. 50 is a transverse section through the upper part of the region of
the sensory niche, not quite horizontal (_i. e._ parallel with the bell
margin), but slanting so as to lie on the plane of the reference arrow
_x-y_ in Fig. 49. The plane passes just through the top of the niche, and
in two areas has cut through the roof with its epithelium of ectoderm
(_ece_, _ecs_) so that the space of the sensory niche (_sn_) appears. The
vascular lamella of the sensory niche (_vls_) is shown, as in Figs. 13
and 14, running on each side from the endoderm that lines the canal of
the sensory club (_enc_) to the endoderm of the adjacent stomach pocket
(_enp_). By it the gelatine of the exumbrella is separated from that of
the subumbrella, and one sees that it is only through the latter that
the nerve has to pass in order to reach the base of the sensory club. It
is also seen that one part of the roof of the niche which is cut through
lies outside of the ring of lamella and is therefore lined with ectoderm
of the exumbrella (_ece_) while the other lies within the ring and is
lined with ectoderm of the subumbrella (_ecs_). Owing to the slanting
direction of the cut only the root on one side is cut through. The other
is indicated, however, on the right side of the drawing. In this method
of passage of nerve fibres, together with the accompanying ganglion
cells, directly through the gelatine to the stalk of the sensory club my
work is only confirmation and explanation of Claus.
Fig. 51 is a vertical section through the base of the stalk in the plane
of the reference arrow _w-z_ in Fig. 49, and therefore passing through
one of the roots of the nerve of the stalk. Here again the region is seen
to be cut off from the exumbrella by the vascular lamella of the sensory
niche (_vls_), and the nerve is seen passing through the gelatine of
the subumbrella from the surface of the bell cavity (_sc_) to the base
of the stalk hanging in the sensory niche (_sn_). One of the ganglion
cells (_gc_) that accompany the nerve is seen to have two nuclei, a not
infrequent occurrence which has been pointed out by others.
The same figure shows that the axis (_ax_) of the nerve has penetrated
the gelatine with the other fibres. Here at the base of the stalk it
takes a horizontal course and becomes directly continuous with the
similar structure of the other root, as Wilson, I believe, first pointed
out. This part of the nervous tract which runs horizontally along the
base of the stalk between the two roots (Fig. 49, _rns_) has been
considered by Claus the representative in Charybdea of the upper nerve
ring of the Craspedota, which therefore exists in Charybdea in four
separate portions. Seeing, however, that the region in which it is found
belongs to the subumbrella, the homology seems very doubtful. Moreover,
the fact that the axis of the nerve ring runs through this outer portion,
instead of remaining on the inner surface of the subumbrella and passing
to the radial ganglion, rather indicates that the outer portion is part
of the original course of the nerve ring, while the portion that remains
on the inner surface is perhaps a later formation.
A very interesting feature of the nervous system occurs in the same
region in the form of a tract of fibres underlying the endoderm, and
separated from the other fibres by the gelatine of the supporting
lamella. It is seen in vertical section in Fig. 52 (_enf_), which
is a section through the base of the stalk in just about its median
plane, and, therefore, to one side of the arrow _w-z_ in Fig. 49 and
the corresponding drawing, Fig. 51. In cross-section it is represented
also in Fig. 50 (_enf_). It varies in size and prominence very much
in different specimens. Fig. 52 is a camera drawing of it in the case
that showed it most developed. Ganglion cells are found in it, but
comparatively infrequently. In some cases the tract itself can hardly
be found with certainty. Hesse has described in a Rhizostome a much
more highly developed tract in a corresponding position on the base of
the marginal body. Fibres from the “outer sensory pit” pass through the
gelatine to the sub-endodermal tract, which is described as surrounding
the epithelium of the canal of the marginal body like a collar and is
most thickly developed on the under surface of the canal, at the place
that just corresponds with the point where, and where only, I find the
tract in Charybdea. Hesse thinks that fibres then pass from this region
to the nervous epithelium of the “inner sensory pit” lying underneath
the base of the marginal body, which contains a rich supply of ganglion
cells and is considered by him to be the centre of the nervous system of
the medusa. A close comparison cannot be drawn with Charybdea in this
matter, however, since Charybdea has nothing to correspond with the
“outer” and “inner” sensory pits. Moreover, the endodermal tract is not
found encircling the canal of the sensory club, nor could I trace fibres
passing from it through the supporting lamella into the fibres of the
nerves.
Claus has figured (’78, Taf. V, Fig. 45, _Fb_) a small bundle of fibres
in the stock of the sensory club lying between the endoderm cells of
the canal and the supporting lamella. The same bundle is found in both
Charybdea and Tripedalia and can be traced in cross-sections up the
stalk to a point which must correspond with that at which the endodermal
tract is seen in Fig. 52. Downwards it can be traced only as far as the
entrance of the stalk into the knob of the club where it invariably
becomes lost to view. According to Hesse (’95, p. 427) Schäfer found
under the endoderm cells of the whole stalk of the marginal body a
fibrous layer like that under the endoderm cells which he refers to
slender processes from the cells of the crystalline sac. Although Hesse,
as we have seen, finds the layer more limited in extent than Schäfer
gives it, and does not trace it to the same source, the observation of
Schäfer seems to me worthy of mention here, inasmuch as the trend of
the fibrous bundle under the endoderm cells of the stalk in Charybdea
and Tripedalia suggests quite strongly that the fibres come from the
crystalline sac, as Schäfer thought to be the case in his medusa.
Besides the radial ganglion situated in the course of the nerve ring at
its four perradial points there are four other similar ganglia on the
subumbrella. These lie in the interradii, at the four lowermost points
of the nerve’s course, and undoubtedly send off nerves into the pedalia
at whose bases they are situated. F. Müller (’59), whose work was not
accessible to me, is quoted by Claus as recording two ganglia opposite
the base of each pedalium which gave off a great number of nerves partly
into the velarium, partly into the tentacles. Claus observed nothing of
the kind in Charybdea and states that even the interradial ganglia do not
exist.
That they do, however, is shown without doubt in sections of both C.
Xaymacana and Tripedalia, but nerves to the velarium or to the tentacles
I was unable to find.
On the two sides of each frenulum and of each suspensorium are found
subepithelial ganglion cells in greater numbers than elsewhere on the
subumbrella, and I am inclined to ascribe to them also the importance
of special ganglia controlling the musculature of the frenula and
suspensoria. Certainly such ganglia would not be out of place.
It has been mentioned that the greater prominence of the radial nerve and
the possession of special sensory organs in the proboscis were the only
points of difference I had noted between the nervous systems of Charybdea
and Tripedalia. These sensory organs remain to be described. They are
simple ciliated cysts containing a concretionary mass, and are situated
in the gelatine of the proboscis, irregularly disposed of at any level,
from the lips to the beginning of the stomach, and in any radius. In one
series of the adult animal fifteen were counted, of which seven were
situated about interradially, four perradially, two adradially and two
subradially. In another, twenty-one were counted, twelve in the perradii
and nine situated between the sub-and perradii. The one shown in Fig. 24
is in the perradial position, often seen. In the sections of the very
young Tripedalia in which the vascular lamella had not reached the adult
condition the sensory organs of the proboscis were not found, although
the sensory clubs showed practically no difference from the adult.
Their structure is very simple--merely a round or oval sac lined with
ciliated cells which bear up and keep in constant motion an irregular
coarsely granular concretion. Fig. 53 is a sketch made in Jamaica from
the living specimen. Sections were somewhat disappointing in that they
added but little. Fig. 55 was drawn to show that now and then a mucous
cell (_mc_) is found among the other cells of the sensory epithelium. An
irregular-shaped mass (_rc_) was always found inside the cysts as the
organic remains of the concretion. It gave no trace of cellular structure
and offered no evidence whether the concretion was the product of one or
few or of all the cells of the cyst. The latter would be unique among
the medusæ. Even if the otocyst is the result of the activity of only
one or a few cells, it is, so far as I know, the only case known for the
jelly-fish of a free, unsuspended concretion.
As to whether the cysts are of ectodermal or endodermal origin could not
be determined, but there was some evidence in favor of the latter. Fig.
56 is a drawing of one seen in optical section in a whole mount of part
of a proboscis, and shows a definite connection with the endoderm of the
proboscis. This was the only case when such connection was satisfactorily
established, but in sections it was not uncommon to find what seemed to
be the remains of the broken stalk, as in Fig. 54 (_rs?_). No connection
could be traced between the cysts and any other part of the nervous
system. As to function, the idea that they serve to give perception of
space relations suggests itself as readily as any other hypothesis.
We come now to the consideration of the terminal knob of the clubs, the
sensory portion proper. A complete and detailed account of the complex
structure of these organs would fill many pages and involve much useless
repetition. Claus (’78) has described them with accuracy, but not in
great detail, and since then Schewiakoff (’89) has given a careful
general description and has supplemented Claus’s work by observations
upon the finer structure made with the aid of more recent technique. It
seems in place for me, therefore, to give in the briefest possible way a
general idea of their structure, and to pass then at once to the points
in which my work has led me to different conclusions from those of Claus
and Schewiakoff. In brief, then, the knob of the sensory club consists
of a thick, complex mass of nerve fibres, more or less imbedded in which
lie the special sensory organs, surrounding the ampulla-like terminal
enlargement of the canal. The surface between the special organs is
covered with less specialized sensory epithelium. The sensory organs are
seven in number. Of these, four are simple invaginations of the surface
epithelium arranged in two pairs symmetrically to the median line in the
proximal end of the knob (the end where the stalk enters) and having
pigment developed in the cells so invaginated, while the space of the
invagination is filled with a gelatinous refracting secretion. These
are considered simple eyes. Two more of the organs are complex eyes
situated on the median line of the inner surface of the knob, the upper
one smaller than the lower, but having almost exactly the same structure.
Each has a cellular lens over which extends a superficial, corneal layer
of cells; below the lens a refractive “vitreous body”; and below this
a retina with pigmented cells. The seventh organ is the crystalline
sac, which lies almost at the end of the knob opposite to the stalk and
contains a large concretion. In view of the fact that the sensory clubs
_in toto_ have been abundantly figured by Claus and Schewiakoff, it is
my intention to give but one simple figure of the general relations, and
I justify that one in that it was made from the fresh material. Fig.
57 is a camera sketch of the outlines given by a sensory club seen in
optical section from the side. The smaller upper and the larger lower
complex eyes which are situated on the mid-line, are seen in profile,
while the two small simple eyes give the outlines that they would in a
surface view of their side of the knob. Of course it is understood that
two similar ones would appear on the other side, since the four simple
eyes are symmetrically paired on either side of the mid-line. The sketch
seems to show at least this much, that even in the living state the lens
of the larger eye projects out beyond the other contours of the surface,
so that the marked convexity ascribed to it in descriptions is not to be
attributed to the preservation.
It is in reference to the structure of the retina and vitreous body of
the complex eyes that I have found myself unable to come to the same
conclusions as Claus and Schewiakoff. Since the work of the latter goes
much further into the detail of the subject than does Claus’s paper, it
will be sufficient for me to compare my results simply with those of
Schewiakoff.
The latter finds that the retina is composed of two kinds of cells,
corresponding to the supporting and sensory cells referred to in the
description of the nerve ring. These he figures (’89, Taf. II, Figs.
12 and 13) as alternating regularly. The two kinds of cells differ as
follows:
(1) Shape. The supporting cells like those referred to before, are
cone-shaped, having a proximal fibrous process that runs into the
underlying stratum of nerve fibres, and on the surface of the retina a
broad distal pigmented termination. The sensory cells are spindle-shaped,
the proximal processes becoming continuous with fibres of the underlying
nervous mass, while the distal process runs up to the surface of the
retina (the part toward the lens) in between the ends of the supporting
cell. The two kinds of cells are accordingly designated as pigment and
visual.
(2) Position of nucleus. This comes in as a corollary of the shape.
The nuclei of the visual cells lie in the enlarged central part of the
spindle-shape, and, therefore, at a lower level than the nuclei of the
alternating pigment cells.
(3) Processes in the vitreous body. The distal processes of the
spindle-shaped visual cells are continued through the vitreous body to
the cells of the lens as rod-like visual fibres which lie in canals in
the (supposedly) homogeneous vitreous body. The pigment cells on the
other hand have no fibres passing from them through the vitreous body,
but in the latter are situated cone-shaped masses of pigment whose bases
rest upon the broad ends of the pigment cells without, however, being a
part of the cell.
(4) Pigment. The distal ends of the pigment cells in the retina are
strongly pigmented, as the name implies. The processes of the visual
cells, which alternate with these, are pigmented likewise, but the
pigment is not so abundant and lies in the periphery of the cell body,
leaving free a highly refracting central axis.
If the relation of these cells to each other has been made sufficiently
clear, it will be understood that, in accordance with Schewiakoff’s
scheme of the structure, sections that cut the retinal cells transversely
give very different appearances at different levels. A section through
the very tops of the retinal cells, that is, the last section of the
retina before striking the vitreous body, would show large polygonal
areas of heavy pigment (the ends of the pigment cells), in between which
would lie the much smaller, less pigmented, highly refracting ends of the
visual cells (’89, Taf. II, Fig. 19). A section lower down in the retina,
that is, more toward the centre of the club, would strike the low-lying
enlarged central portion of the visual cells with their contained nuclei,
and the smaller, proximal ends of the pigment cells. It would, therefore,
give the reverse appearance from the preceding section, namely, that of
large unpigmented (or but slightly pigmented) areas (the swollen bodies
and nuclei of the spindle-shaped cells), and in between them smaller
pigmented areas, the ends of the proximally tapering pigment cells
(’89, Taf. II, Fig. 20). A section on the other side of the one first
described, that is, one of the first through the vitreous body, would
show pigment areas of the same size as the large ends of the pigment
cells (the cone-shaped streaks of pigment in the vitreous body which
according to Schewiakoff are associated with the pigment cell), and in
between them the cross-sections of the rod-like processes from the visual
cells, lying in canals in the clear homogeneous ground-substance of the
vitreous body (’89, Taf. II, Fig. 18).
Let me give a resumé of Schewiakoff’s conception of the structure of the
retina.
a. There is an alternation of pigment and visual cells, the nuclei of
the spindle-shaped visual cells lying at a lower level than those of the
cone-shaped pigment cells.
b. From the visual cells extend rod-like processes into the vitreous
body, lying in canals in the latter.
c. In the vitreous body a cone-shaped streak of pigment overlies each
pigment cell of the retina, which is not a part of that cell.
d. Apart from these pigment streaks and the rod-like processes of the
visual cells the vitreous body is structureless, probably a secretion of
the pigment cells.
My own work, now, has led me to a different conception, so that my
conclusions on the same points would be as follows:
a. There is not good evidence of an alternation of cone-shaped pigment
cells and spindle-shaped visual cells, with the nuclei of the latter at a
lower level than those of the former.
b. From some of the retinal cells otherwise not distinguished, there
extend rod-like processes into the vitreous body, such as described by
Schewiakoff.
c. The cone-shaped streaks of pigment in the vitreous body belong to the
underlying pigment cells, in fact are direct continuations of them, and
at their distal ends they are prolonged into fibrous processes lying in
canals of the vitreous body exactly like the visual fibres of Schewiakoff.
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.
[Illustration: [Dr. Conant did not complete Fig. 72, and the
accompanying outline of Fig. 7 of Schewiakoff’s memoir (Beiträge zur
Kenntnis des Acalephenauges, Morph. Jahrb., Bd. XV, H. 1) has been
substituted.--EDITOR.]
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.)
[Illustration: CUBOMEDUSÆ. PLATE I.
Gilman Drew, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE II.
Conant & Crew, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE III.
F.S. Conant, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE IV.
F.S. Conant, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE V.
F.S. Conant, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE VI.
F.S. Conant, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE VII.
F.S. Conant, del. Heliotype Co., Boston.]
[Illustration: CUBOMEDUSÆ. PLATE VIII.
F.S. Conant, del. Heliotype Co., Boston.]