A: CHARYBDEA XAYMACANA.
a. _Environment and habit of life._
1. The Cubomedusæ are generally believed to be inhabitants of deep water
which come to the surface only occasionally. Both of the Jamaica species,
however, were found at the surface of shallow water near the shore,
and only under these circumstances. Whether these were their natural
conditions, or whether the two forms were driven by some chance from the
deep ocean into the Harbor and there found their surroundings secondarily
congenial, so to speak, can be a matter of conjecture only. C. Xaymacana
was taken regularly a few yards off-shore from a strip of sandy beach
not ten minutes row from the laboratory at Port Henderson. It was seen
only in the morning before the sea-breeze came in to roughen the water
and to turn the region of its placid feeding-ground into a dangerous
lee-shore. Some of the specimens taken contained in the stomach small
fish so disproportionately large in comparison with the stomach that they
lay coiled up, head overlapping tail. The name Charybdea, then, from
the Greek χαρύβδις (a gulf, rapacious), seems to be no misnomer. It is
worth mentioning that the digestive juices left the nervous system of the
fish intact, so that from the stomach of a Charybdea could be obtained
beautiful dissections, or rather macerations, of the brain, cord, and
lateral nerves of a small fish.
In size C. Xaymacana agrees very well with the average of the genus.
The four single tentacles characteristic of the genus are very
contractile, varying from two or three to six or seven inches in length,
and probably if measurements could be taken while the animal was
swimming freely about, the length would be found to be greater still.
Charybdea is a strong and active swimmer, and presents a very beautiful
appearance in its movements through the water, the quick, vigorous
pulsations contrasting sharply with the sluggish contractions seen in
most Scyphomedusæ. With its tentacles streaming gracefully behind, an
actively swimming Charybdea presents a fanciful resemblance to a comet
or meteor. When an attempt is made to capture one, it will often escape
by going down into deeper water--as indeed do other jelly-fish. Escape
from observation is all the more easy by reason of the entire absence
of pigment excepting for the small amount in the sensory clubs. The
yellowish or brownish color usually stated as common in the Cubomedusæ is
nowhere present in C. Xaymacana.
b. _External Anatomy._
2. _Form of Bell._ C. Xaymacana shows the typical division of the
external surface into four almost vertical perradial areas (Figs.
1-3, _p_), separated by four stoutly arched interradial ribs or bands
(Figs. 1-3, _i_). These ribs thus play the part of corners to the
Cubomedusan pyramid. They are formed by the thickenings of the jelly of
the exumbrella, and serve to give the necessary strength to the four
interradial corners, each of which bears one of the four tentacles at
its base. Each rib is further divided into two longitudinal strips by a
vertical furrow lying exactly in the interradius (Fig. 2, _ifr_). The
surface of the exumbrella is thus marked by twelve longitudinal furrows,
as seen in the same figure (2). Of these, four are the interradial
furrows just mentioned; the other eight are the adradial (_afr_)
furrows, which set off the four perradial surfaces of the pyramid from
the four interradial ribs or bands of the corners, each of which is
again subdivided, as mentioned above, by the shallower interradial
furrows. Each interradial furrow ends above the base of the corresponding
pedalium, at about the level of the sensory club; each adradial furrow
diverges toward the perradius in the lower third of its course, and thus
with its companion furrow narrows down the perradial surface of the
pyramid in the lower part of the bell to an area of not much greater
width than the niches in which the sensory clubs lie. The projecting
interradial corners are of course correspondingly enlarged in the lower
part of the bell, and in this way the contours of the surface are changed
from those figured in the view of the bell from above (Fig. 2) to those
of Fig. 3, which represents a view of the bell margin from below.
3. _Pedalia._ From the base of the interradial corner bands spring the
four pedalia (Fig. 1, _pe_), gelatinous appendages of the margin having
much the same shape as the blade of a scalpel. These in turn bear on
their distal ends, as direct continuations, the long, contractile, simple
tentacles. The relatively stiff pedalia have the same relation to the
flexible tentacles that a driver’s whip-stock has to the long lash.
In the living animal the pedalia are found attached to the margin at
an angle of about 45° with the longitudinal axis of the bell. In the
preserved specimens they are bent in toward the axis by the contraction
of the strong muscles at their base, in which position they are figured
by Claus for C. marsupialis (’78, Taf. I., Figs. 1 and 2).
The pedalia are in reality processes belonging to the _subumbrella_, as
will be shown in the section treating of the vascular lamella. They are
composed chiefly of gelatine covered with thin surface epithelium and
carrying within the gelatine the basal portion of the tentacle canals.
They have received various names at the hands of the writers. Gegenbaur
called them “Randblätter.” Claus gave them the name of “Schirmlappen,”
and incorrectly homologized them with the marginal lobes of other
Acraspeda. Claus’s error was corrected by Haeckel, who termed them
“Pedalia” or “Gallertsockel,” and homologized them with the pedalia of
the Peromedusæ. Besides furnishing a base of support for the tentacles
they may perhaps also serve as steering apparatus, a function for which
their thin blade-like form would be admirably adapted.
Internal to the base of each pedalium, between it and the velarium, is
found a funnel-shaped depression of the ectodermal surface. This is
shown in Fig. 5 (_ft_) in longitudinal section, and in cross-section in
Fig. 16. In the latter figure the epithelium of the outer wall of the
funnel (_mt_) is shown much thickened, the result of a stout development
of muscle fibres. These are the muscles that in the preserved specimens
cause the inward contraction of the pedalia referred to above.
4. _Sensory Clubs_ (marginal bodies, rhopalia). In spite of their
position above the bell margin, the four sensory clubs, representing as
they do transformations of the four perradial tentacles, are properly
classed with the pedalia and interradial tentacles as appendages of
the margin. They lie protected in somewhat heart-shaped excavations or
niches in the perradial areas of the exumbrella. Each sensory niche is
partially roofed over by a covering scale, a hood-like projection from
the exumbrella. Below the covering scale the water has free access to the
niche and to the sensory club within it. The sensory club consists of a
hollow stock directly homologous with tentacle and canal, and a terminal,
knob-like swelling, the sensory portion proper. The latter contains
on its inner surface--the surface turned towards the bell cavity--two
complicated unpaired eyes with lens, retina, and pigment, lying one above
the other in the median line; and at the sides of these, two pairs of
small, simple, pigmented, bilaterally symmetrical eye spots. At the end
of the club, that is, on its lowermost point, lies a sac that contains a
concretion and is usually considered auditory. The canal of the stalk is
directly continuous with the gastro-vascular system. In the swollen knob
of the sensory club it forms an ampulla-like terminal expansion.
As was pointed out by Claus, the bottom of the sensory niche--by bottom
is meant the vertical wall that separates the space of the niche from
the bell cavity--is formed from the subumbrella only. This arrangement
of parts, apparently impossible for a structure so far removed from
the bell margin as the sensory niche, will be explained more fully
under the special topic of the vascular lamellæ, or cathammal plates.
It is sufficient at this point to refer to Fig. 44, which shows the
shield-shaped area mapped out by a vascular lamella that connects the
endoderm of the stomach pocket with the ectoderm of the bottom of the
niche. By this the exumbrella is completely cut off from any part in the
formation of the bottom of the niche. Cross and vertical sections through
the niche (Figs. 39 and 37) help to a better understanding of these
relations. Since the base of the stalk of the sensory niche lies within
the ring of vascular lamella, the whole organ as well as the bottom of
the niche belongs to the subumbrella, and so in spite of its position
some distance upwards from the bell margin the sensory club is very
properly called a “marginal body” (Randkörper).
The epithelium of the sensory niche consists entirely of the
flattened ectodermal surface layer common to the whole exumbrella. No
differentiation suggestive of nervous function in addition to that of the
sensory clubs can be discovered, although it would be quite natural to
expect to find something of the sort, as intimated by Claus (’78, p. 27).
It is worth while to mention again the fact that the eyes are directed
inwardly toward the cavity of the bell. The larger and lower of the two
median eyes looks into the bell cavity horizontally; the smaller upper
eye is turned upward toward the region of the proboscis. This is in the
normal pendant position of the sensory club. The stalk, however, is
very flexible, and a range of other positions of the sense organs is
possible, although nothing was observed to suggest that such positions
were within the control of the animal. The eyes evidently have as their
chief function to receive impressions of what is going on _inside_ the
bell, not outside. Perhaps the strongly biconvex, almost spherical lenses
of the median eyes also point to a focus on near and small objects.
5. _The Bell Cavity and its Structures._ In general, the bell cavity
repeats the external form of the bell, being almost cubical. In
cross-section it appears very nearly square with the angles in the
interradii as seen in the series of drawings that figure sections of the
whole jelly-fish at different levels (Figs. 6-16). Above, the bell cavity
is roofed over by the stomach; below, it is open freely to the water,
the opening being narrowed somewhat by the diaphragm-like velarium (Fig.
3, _v_); the four flat perradial sides are bounded by the walls of the
four broad stomach pockets, to be described when we come to the internal
anatomy.
(a) _The Proboscis._ From the stomach there hangs down into the
bell cavity the proboscis or manubrium, which consists of a short
funnel-shaped stalk bearing on its distal end the four mouth lobes or
lips. The latter are somewhat broadly V-shaped processes lying in the
perradii with the convexity directed outwards, and with the concavity
on the inside forming the beginnings of four perradial furrows that are
continued upwards to the stomach. The four furrows are shown in the stalk
of the proboscis in Fig. 11, which represents a section taken a little
above the level of the mouth lobes. The same cross-shaped section of
the stalk shows the four perradial prominences or ridges overlying the
furrows, which are the direct continuations of the four projecting mouth
lobes.
(b) _The Suspensoria or Mesogonia._ The stomach (leaving out of
consideration the proboscis) hangs down into the bell cavity as a
slightly sagging saucer-shaped roof (Figs. 4 and 5). In the four perradii
it is attached to the lateral walls of the subumbrella by four slenderly
developed mesentery-like structures, the suspensoria or mesogonia. These
are simple ridges of gelatine, covered of course with the epithelium
of the bell cavity, which serve to keep the stomach in position much
in the way that a shelf is supported by brackets (Fig. 4, _su_). The
suspensorium accordingly has two parts, curved so as to lie at right
angles with each other: a vertical portion lying along the wall of the
subumbrella, and a horizontal which passes over from the vertical on
to the basal wall of the stomach. In Fig. 10 the suspensorium in each
quadrant is shown cut across just below the angle between the two parts,
so that the two appear in the section as projections on the wall of the
stomach and on the wall of the subumbrella.
(c) _The Interradial Funnels or Funnel Cavities._ It will be seen at
once that the four suspensoria serve as partitions to divide the upper
portion of the bell cavity, the part that lies between the stomach
and the lateral walls of the subumbrella, into four compartments.
These compartments extend upwards in the four interradii like inverted
funnels, whence their name. In the series of cross-sections they can be
traced upwards with constantly diminishing area from the level of the
suspensoria, Fig. 10 (_if_), to Fig. 6, which is taken very near the top
of the bell. Homologous structures exist in all the Scyphomedusæ, and in
some of the Lucernaridæ they are continued up even into the stalk of the
attached jelly-fish.
(d) _The Velarium._ Charybdea, like most of the Cubomedusæ, possesses a
velum-like structure around the opening of the bell cavity (Fig. 3, _v_).
The velarium is a thin muscular diaphragm, resembling the true velum in
position and essential structures, but differing from the velum in its
origin, and in the possession of diverticula from the gastro-vascular
system, the velar canals. Of these there are in C. Xaymacana very
regularly sixteen, four in each quadrant. Their outline is seen in Fig.
3 to be forked with small irregular accessory processes. As for its
origin, the velarium of the Cubomedusæ is commonly accounted to have
arisen by fusion of marginal lobes, as in the case of the velarium of the
Discomedusæ. Pending decisive ontological evidence, the slight notches
in the four perradii seen in Fig. 3 may perhaps be taken as slight
indications of a primitive unfused condition, but the question will be
brought up again when the vascular lamellæ are discussed.
(e) _The Frenula._ Just as the stomach is attached to the walls of
the subumbrella in the four perradii by the suspensoria, so in the
lower part of the bell cavity the velarium is attached to the wall
of the subumbrella in the perradii by four structures similar to the
suspensoria, the frenula velarii. The frenula, like the suspensoria,
resemble the brackets of a shelf, with the difference that in the case of
the frenula the bracket is above the shelf, their purpose being evidently
to keep the velarium stiff against the outflow of water produced by
the pulsations of the bell. According to the greater need of strength
in this case, we find the frenula stouter, more buttress-like than the
suspensoria. The gelatinous ridge that gives them the necessary firmness
is thickened so as to be triangular in section, as shown in Fig. 16
(_frn_).
(f) _Musculature._ As is general in medusæ, the muscular system, so
far as known, is restricted to the subumbrella. It has a very simple
arrangement, consisting of a continuous sheet of circular (_i. e._
horizontal) striated fibres, which is interrupted only in the four
perradii by the radially directed muscle fibres of the suspensoria and
the frenula. In each quadrant, between the muscle of the suspensorium
above and that of the frenulum below, in an area just internal to the
sensory niche, there lies a space free from muscle. This interruption
of the muscle layer is shown in Fig. 39. Under the head of musculature
belonging to the subumbrella must be included also the radial, or
longitudinal muscles at the bases of the pedalia, which were mentioned
before (Fig. 16, _mt_). The mouth lobes and proboscis also are highly
contractile and muscular.
(g) _Nerve Ring._ It is in the possession of a clearly defined nerve ring
that the Cubomedusæ differ from all other Scyphomedusæ whose nervous
system has been carefully studied. The nerve ring shows very plainly
on the surface of the subumbrella as a well-defined clear streak. Its
course is zig-zag or festoon-like. In the interradii, at the basis of the
tentacles, it lies not far from the bell margin. In the perradii it rises
to the level of the sensory clubs. This very striking arrangement is
understood at once when it is remembered that the sensory clubs represent
the four perradial primary tentacles, and were originally situated on
the margin. When all the rest of the margin grew down and away from the
four sensory clubs, fusing below them to form the present intact edge of
the bell, the four portions of the nerve ring that lay in the perradii
were left at the level of the sensory clubs, and the originally straight
nerve ring was thus bent into a bow in each quadrant. The finer structure
of the nerve will be treated of in the special part to be devoted to the
nervous system.
c. _Internal Anatomy._
6. _Stomach._ The shape of the stomach is approximately that of a
biconvex lens, as seen in Fig. 4, which represents a Charybdea cut in
halves longitudinally in the perradius. The lumen of the proboscis (the
buccal stomach according to Haeckel’s terminology) communicates directly
by a funnel-shaped enlargement with the stomach proper, or central
stomach of Haeckel. The term basal stomach is carried over by Haeckel
from the Stauromedusæ, where it has considerable significance, to the
Cubomedusæ, and applied to the upper part of the central stomach. In the
stalkless Cubomedusæ, however, it has no significance so far as actual
structure goes, and our knowledge of the development of the Cubomedusæ is
as yet too simple for us to say that the upper part of the main stomach
represents what remains of the basal stomach of an earlier pedunculated
stage.
The epithelium of the roof of the stomach is not specially differentiated
and apparently has little or no part in digestion. The epithelium of the
floor, on the other hand, is composed chiefly of very high and thickly
crowded columnar cells which are usually described as coarsely granular,
but under high powers appear to be filled with vacuoles surrounded by
a network of cell substance. Thickly interspersed among these columnar
cells are goblet cells filled with mucus. The floor is thrown into
numerous wrinkles by ridges in the supporting gelatine resulting in
increase of digestive surface. The four perradial grooves of the
proboscis are continued in the perradii along the floor of the stomach
as four fairly deep furrows, which lead directly to the gastric ostia
and stomach pockets--structures to be described presently. These furrows
are lined with crowded columnar cells, smaller and denser than the other
cells of the digestive epithelium, containing no granules and but little
beside the relatively large, compact, deeply staining nuclei. The furrows
probably represent special ciliated courses.
7. _Phacelli._ Lying in the four interradial corners of the stomach are
the four phacelli or tufts of gastral filaments to the number of thirty
or thirty-five in each tuft. The filaments are attached to a single
stalk, like the fringe of an epaulette or the hairs of a coarse brush.
The stalk bearing the filaments is an outgrowth of the lower wall of the
stomach just at the point where it fuses with the upper. The phacelli are
therefore structures of the subumbrella, proof of which will be found
under the special topic of the vascular lamellæ. The stalk, an indication
of which appears in _sph_. Fig. 6 (the section being a little below the
axis of the stalk, which lies horizontally), consists of a firm core of
gelatine covered with the high columnar epithelium of the floor of the
stomach. The filaments themselves are slender processes repeating the
structure of the stalk and having a central axis of gelatine for support
covered with glandular epithelium, which in the case of the filaments
bears numerous nettle cells. These processes are extremely contractile,
and in the living animal show a continuous, slow, squirming movement
like a mass of worms. The section just referred to (Fig. 6) shows
diagrammatically three of these filaments (_fph_) cut across in each
quadrant.
8. _Peripheral Part of the Gastro-vascular System._ The proboscis and
stomach proper comprise the central part of the gastro-vascular system.
In direct communication with the central is a peripheral part composed of
pouches or pockets lying in the vertical sides of the cube-shaped bell,
just as the central stomach lies in its roof. The peripheral part may be
subdivided for convenience of description into the stomach pockets, the
marginal pockets, and the canals of the tentacles and sensory clubs.
(a) _Stomach Pockets._ These are four broad, thin pouches lying between
the exumbrella and the subumbrella in the four perradii (_e. g._ Fig.
9, _sp_) and separated from one another in the interradii merely by four
thin vertical strips of vascular lamella (_ivl_) or fusion between the
two endodermal surfaces of a primitively single undivided peripheral
cavity. The structure is exactly that which we should have if in a
Hydromedusa, for example Liriope (Trachomedusæ), the four radial canals
broadened out and the intervening cathammal plates correspondingly
narrowed, until the relations in size were just reversed, and instead
of four narrow radial canals separated from one another by four broad
cathammal plates, we had four broad radial canals or pouches separated by
four narrow cathammal plates.
The stomach pockets communicate at their top with the central stomach by
means of four moderately large openings, the gastric ostia. These are
seen in a side view of the whole animal as triangular spaces (Fig. 1, _g.
o._) near the top of the broad perradial sides. In Figures 7 and 8 they
are seen in cross-sections, in Fig. 4 in vertical section.
The communication between the stomach and each stomach pocket is guarded
by a valve that can cut the one entirely off from the other. The valve is
simply the flexible lower margin of the gastric ostium, a thin vertical
fold of the floor of the stomach, semilunar in shape, just at the point
where it is passing over into the stomach pocket. A longitudinal section,
such as is shown in Fig. 4, gives the best idea of the form and position
of the valve that can be obtained from any simple section. Internal to
the valve is seen a depression of the stomach wall, almost worthy to be
called a pocket. The valve itself lies as a wall across the end of this
depression, obstructing a free course to the stomach pocket. It will be
seen at once that any pressure of fluids in the stomach against this
vertical wall, or valve, would serve only to press it against the inner
surface of the exumbrella, and thus effectually close the entrance into
the stomach pocket. Such a closure would both keep the juices of the
stomach from entering the pockets and the embryos in the pockets from
entering the stomach before the proper time.
The depression of the floor of the stomach just internal to the valve may
possibly be a structure of some morphological significance. In one series
of sections it was found that in two of the quadrants the depression was
deeper than that represented in Fig. 4, and extended perceptibly into
the outer or vertical portion of the suspensorium. Fig. 32 is a diagram
giving a vertical reconstruction in the perradius of the cross-sections
in which this deepened depression was noticed. Fig. 31 is a drawing
(the outline by camera lucida) of one of the cross-sections, through
the lowermost point of the depression. The figure gives the wall of
the stomach lined with high columnar epithelium (_ens_), and the wall
of the stomach pockets, with the suspensorium (_su_) connecting them.
The section is taken just above the broad angle that lies between the
two parts of the suspensorium, that is, in a plane parallel to the arrow
_a-b_ in Fig. 32, but a little lower down. At the points to which the
reference letter _x_ (Fig. 31) refers are seen the first indications of
the division into two parts, _i. e._ of the apex of the angle. The next
section or two lower down show the relation seen in Fig. 10 (_su_). There
can be no doubt in this case that the depression or pocket lies in the
outer vertical limb of the suspensorium. It is the position that gives
it at least the appearance of some morphological significance. In two
genera of Lucernaridæ named and described by Clark (’78), Halicyathus
and Craterolophus, the mesogonia or suspensoria in all four perradii
contain broad pockets. These mesogonial pockets in the Lucernaridæ have
given rise to considerable misunderstanding owing to the fact that in
some forms the reproductive organs bulge out from the stomach pockets in
which they structurally lie, and come to take up a secondary position
in the walls of the mesogonial pockets. The sections of Charybdea above
referred to indicate that among the Cubomedusæ we may have the same
structure reduced to its lowest terms, and may be a feather’s weight in
favor of the view that the Cubomedusæ are descendants of an attached
Lucernaria-like form.
Two more diagrams, Figs. 33 and 34, are added in order to give a
more complete understanding of a gastric ostium and its neighboring
structures, the mesogonial pocket and the valve. Fig. 33 is a view of
the gastric ostium and valve from the stomach side, and represents the
appearance that would be given by a thick section through the arrow
_x-y_ in Fig. 32, in a plane at right angles to the paper. The heavy
lines outlining the gastric ostium (_enr_ and _enfl_) represent the
place where the plane of the section has cut across the epithelium of
the roof of the stomach above the ostium and the epithelium of the floor
of the pocket-like depression internal to the valve. The continuation of
the two heavy lines in either side of the ostium represents the region
where the roof and floor of the stomach meet; _i. e._, the edge of the
lens-shaped stomach. The semilunar outline of the valve (_vg_) is shown
by a light line just above the epithelium of the depression. As is seen
by the reference arrow in Fig. 32, the valve lies a little external
to the immediate plane of the section, and hence it is that its inner
surface is seen in Fig. 33 and not a section of it. The vertical part of
the suspensorium (_su_) is seen in section below the epithelium of the
depression. The reference numbers 1, 2, 3 and 4 denote the same points
in Figs. 32 and 33. Fig. 32 referred to Fig. 33 would lie in a plane at
right angles to the paper through the reference arrow _x-v_ of the latter
figure.
Fig. 34 represents a horizontal section through the gastric ostium at
the level of the arrow _a-b_ in Fig. 32, or arrow _c-d_ in Fig. 33. The
reference numbers 5, 6 and 7, 8 denote similar points in the two figures
33 and 34. Fig. 32 as referred to Fig. 34 is through the arrow _e-f_;
Fig. 33 is through the arrow _c-d_. In the series of cross-sections, Fig.
9 is taken at a level a little below that of Fig. 34, and passes through
the basal part of the valve (_vg_).
(b) _Marginal Pockets._ The part of the peripheral portion of the
gastro-vascular system in each quadrant which is called the stomach
pocket extends downwards as far as the sensory niche. Here by the coming
together of the walls of the exumbrella and subumbrella the space between
them is obliterated (Fig. 15) in the immediate perradius. From the
sensory niche downward to the margin each stomach pocket is thus divided
into two smaller pouches, the marginal pockets (_mp_). In each side of
the Cubomedusan cube there are, then, in Charybdea two marginal pockets;
or in all eight, a characteristic of the family Charybdeidæ. The marginal
pockets as the name implies extend downwards to the bell margin, and are
continued into the velarium as the velar canals. Of these (Fig. 3) there
are two from each marginal pocket, or sixteen in all. The constancy in
their number is one of the characteristics that distinguish C. Xaymacana
from the very closely related C. marsupialis of the Mediterranean.
(Compare Fig. 3 with the similar one by Claus for C. marsupialis, ’78,
Taf. I., Fig. 6.) The forked shape, while to be sure the common form in
C. marsupialis, is an almost invariable characteristic in C. Xaymacana.
It may be mentioned again that the presence of these canals is one of the
chief features that distinguish the velarium of the Scyphomedusæ from the
velum of the Hydromedusæ.
(c) _Canals of the Sensory Clubs and Tentacles._ The four interradial
definitive tentacles and the four perradial transformed tentacles, the
sensory clubs, are hollow, and their canals communicate directly with
the peripheral part of the gastro-vascular system. The canal of the
sensory club in each quadrant leads directly out from the stomach by a
somewhat funnel-shaped opening formed by the approximation of the two
walls of the stomach pocket. The relation of the canal of the sensory
club to the stomach pocket is seen at a glance in Fig. 37. It is given
by means of cross-sections in Figs. 12-14. Figure 12 shows the inner
walls of the stomach pocket approaching the outer at two points, leaving
between them a concavity freely open to the rest of the stomach pocket
above and at the sides. Fig. 13, a little lower down, shows the two
walls fused together at two points, making the interspaces a definite
canal communicating with the stomach pocket above only. This canal lies
directly over the sensory niche, and in the next figure (No. 14) the
canal is seen to have passed through the roof of the sensory niche and to
have entered the base of the stalk of the sensory club. In the enlarged
end of the club, the part which bears the sensory structure, the canal
widens into a terminal ampulla-like sac.
The endoderm lining the canal of the sensory club is specially
differentiated. In the stalk it is more columnar than the epithelium
of the stomach pockets, and is made up of cells containing a brightly
staining nucleus with very little trace of cytoplasm. The cell bodies
appear as if filled with a clear, non-staining fluid. Perhaps these cells
give the stalk elasticity to act in connection with the thin layer of
longitudinal muscle-fibres that are found just external to the supporting
lamella. The epithelium of the terminal enlargement of the canal is
composed of very high narrow cells, many of which show two nuclei of
equal size and staining quality lying side by side.
In continuation of the specialized epithelium of the perradial furrows
in the floor of the stomach the inner wall of the stomach pocket shows a
strip of similar densely crowded columnar cells leading from the gastric
ostium downwards to the canal of the sensory club. As in the other case,
the strip probably represents a specially ciliated tract, and perhaps
in it we see the reason why the canal of the sensory club is almost
always found to contain either spermatozoa which are shed by the male
reproductive organs directly into the stomach pocket, or else floating
cells of the kind to be described in the next section.
The canals of the interradial tentacles arise from the peripheral
gastro-vascular system much lower down than those of the sensory clubs,
since these tentacles have preserved their primary positions with
reference to the bell margin. Figure 16 represents a section taken at
the level of the base of the pedalia which gives the connection of the
tentacle canals with the gastro-vascular system. At the level below
the sensory niche the four broad stomach pockets have been divided, as
we have seen, into the right marginal pockets (_mp_). The figure shows
that in the interradial corners the longitudinal septa (_ivl_, in the
preceding figures), or lines of fusion between the two walls of the
peripheral gastro-vascular space, which divide the primitively simple
space into the four stomach pockets, have come to an end, leaving a
connecting canal (_cc_) in each corner as all that remains of the
primitive uninterrupted communication between all parts of the peripheral
system. It is from these four connecting canals that the tentacle canals
take their origin. From this point of origin each tentacle canal passes
downwards, surrounded by the gelatine of the pedalium, into the tentacle
proper.
The connecting canals are of morphological importance in that they are
supposed, with much reason, to represent in the Cubomedusæ the circular
canal of the Hydromedusæ.
9. _Reproductive Organs._ The sexes are separate in Charybdea. In both
sexes the reproductive organs consist of four pairs of long leaf-like
bodies, each leaf attached along one edge to the wall of the subumbrella
in an interradius (see Fig. 1, _r_), and hanging free in the stomach
pockets. From this position in the stomach pockets it is evident that
the reproductive organs are endodermal. The lines of attachment of each
pair is just internal to the longitudinal vascular lamella that fuses the
outer and inner walls of the stomach pockets together in the interradius
(_ivl_), and the reproductive organs are therefore structures belonging
to the subumbrella. It is interesting to note how careful examination
of the medusan organization takes away from the importance of the outer
cup, the exumbrella, and adds to that of the inner, the subumbrella. We
have seen that the phacelli and the sensory clubs, from whose position
it would be supposed that they belonged to the exumbrella, are organs of
the subumbrella, and that there is no muscle-tissue in the exumbrella;
we find now that the reproductive organs belong to the subumbrella, and
it will be shown later that the tentacles, like the sensory clubs, are
structures of the subumbrella also. To the exumbrella are left only the
functions of support and covering.
The mature reproductive organs extend very nearly throughout the entire
vertical length of the bell, and are therefore found in the series of
cross-sections in all but the uppermost and lowermost (Figs. 7-15 _r_).
The organs consist of germ cells within, covered by an epithelium of
columnar cells that shows here and there nettle cells. The ova are
found with different amounts of yolk, according to age, surrounding
a large nucleus almost devoid of chromatin and an intensely staining
nucleolus. In young ova there appears very plainly in every case at
least one small deeply staining body inside the nucleus, which very much
resembles the nucleolus. These are probably so-called yolk nuclei, and
while I have not made a special study of the ovogenesis, I infer that
the constant presence of at least one, points to an origin of the ovum
from a syncytium (of at any rate two cells), similar to that which has
been recently shown by Doflein (’96) to occur in the formation of eggs
in Tubularia. In the nearly mature ovary each ovum is surrounded by a
layer of gelatine, which comes from the gelatinous sheet that enters
the leaf-like ovary for its support along its line of attachment just
internally to the interradial septum. It seems as if the ova, arising
in the epithelium on the surface, pushed their way into the gelatine
inside and there completed their development entirely surrounded by a
slight investment of gelatine, which grows thinner around each ovum as
it increases in size. In males the testes always show a similar division
into compartments by gelatinous meshes, the compartments thus mapped out
being filled with the small brightly staining spermatocytes. Ova and
spermatozoa when mature are set free in the stomach pockets.
10. _Floating and Wandering Cells._ In the stomach pockets, the canals
of the sensory clubs, and even in the stomach itself, are found in
varying numbers freely floating cells having the appearance of young
ova. They vary in size, the smallest being of the size and having the
general aspect of the small ovocytes found in the ovary. The largest
(Fig. 70) have exactly the same structure as the young ovarian eggs
before they have begun to accumulate yolk. The granular deeply staining
cytoplasm, the clear non-staining nucleus with its bright nucleolus and
the nucleolus-like yolk nucleus, all show beyond doubt that these freely
floating cells originate in the ovary.
In some of my preparations these cells are found not only floating free,
but wandering through the tissues. Fig. 70 shows two such wandering cells
fixed just as they were making their way either through the digestive
epithelium into the gelatine of the floor of the stomach, or from the
gelatine into the epithelium. The former seems the more probable, though
why they should want to get into the gelatine is not very easy to
conceive.
Perhaps there is some connection between this and the appearance that the
young ovarian eggs have of pushing their way from the epithelium into
the gelatine of the ovary. And of course it is not impossible that the
whole phenomenon is abnormal, due to rupture of the ovaries which sets
free young ova to exhibit their amœboid tendencies under new conditions.
Against such an explanation, on the other hand, might be urged the fact
that what seem to be the small floating cells are found occasionally
in males as well as females, and that in the females a series can be
traced with a good degree of certainty between the small floating cells
like those found in the walls, and the larger ones which have all the
characteristics of young ova.
However that may be, this amœboid action of cells having the structure of
ova brings to mind the remarkable form of asexual reproduction described
by Metschnikoff for Cunina proboscidea, under the name of “Sporogonie.”
Unfortunately Metschnikoff’s original paper was not accessible to me, so
that I was unable to obtain more particulars on the subject than those
given in Korschelt and Heider’s text-book (p. 33). The reproductive
organs of both males and females of Cunina proboscidea are said to
produce, besides the usual distinctively sexual elements, neutral amœboid
germ cells, which wander into the endoderm of the stomach and circular
canal, and also penetrate into the gelatine of the subumbrella. These
amœboid cells divide parthenogenetically. One of the two cells of the
first cleavage continues to divide and eventually forms an embryo of
Cunina; the other remains amœboid and serves for movement, attachment and
nourishment of the embryo.
Charybdea, however, has shown no sign of any such reproductive process
on the part of its floating and wandering cells. The only indication
that I get as to their use points to a possible nutritive function.
The enlarged terminal portion of the canal of the sensory club almost
invariably contains a number of the small-sized floating cells. These
have a vacuolated, half disintegrated appearance, with the nucleus always
compact and brightly staining. Now, examination of the high columnar
cells that line the enlargement of the canal shows the presence in the
cells of bodies of exactly the same appearance as those in the lumen.
In one case a floating cell was found just at the end of an epithelial
cell, to all appearance half ingested. The identity of the bodies inside
the cells and those in the lumen is shown very clearly in some sections
of material fixed in formalin, which preserves nuclei, cell walls and
general outlines well enough, but does not retain the cytoplasm, and
hence is useless for most purposes of histology. In the endodermal
cells of the terminal enlargement thus preserved are found all the more
distinctly the bright, compact, degenerated nuclei of the ingested cells,
while in the lumen are seen other bright, compact nuclei with the poorly
preserved remains of cell substances around them. In addition to the
evidence from the appearance of the floating cells themselves and their
ingestion by the endodermal cells, a little collateral evidence may
perhaps be brought in from the Tripedalia about to be described. From
the ovaries in this form are detached masses of cells (Fig. 71) which
float free in the stomach pockets among the developing embryos, and to
judge from the vacuolation that appears, are used up in their favor.
These cell masses are described more fully in the part on Tripedalia.
B: TRIPEDALIA CYSTOPHORA.
a. _Habitat._
The species upon which the new family was founded was obtained in great
abundance in one locality in Kingston Harbor in the summer of 1896. The
environment was even more unlike that in which Cubomedusæ have been
found heretofore than in the case of Charybdea Xaymacana. On the west
side of the Harbor there is a part more or less cut off from the main
body of water, and so from the ocean, by a peninsula. This sheltered bay
is dotted with small mangrove islands which toward the head of the bay
become so numerous as virtually to convert it into a mangrove swamp. The
water is shallow and discolored with organic matter, showing that the
tide does not exercise much influence here, and the bottom is for the
most part a black mud, deep enough to make wading very uncomfortable
but not impossible near shore. The islands rise but slightly above the
level of the waters, and the thick vegetation that covers them, for the
most part mangroves, grows out into the water on all sides, forming a
fringe of overhanging boughs. It was here in the shelter of the boughs,
among the roots and half-submerged stems of the mangroves, that the small
Cubomedusa was found to thrive. It could be obtained in great abundance
almost any day, and of all sizes from the largest adults with stomach
pockets filled with eggs or embryos down to small specimens only about
two millimeters in diameter. In but one other place was Tripedalia found,
and that was a similar region of half landlocked water skirted with
mangroves, situated near Port Royal, across the harbor from the locality
just mentioned. It would be hard to find places in which the conditions
of life were more strikingly different from those of the pure deep sea
in which the Cubomedusæ have been generally found before. The slight
brownish yellow pigment made the small medusæ a little difficult to see
in the discolored water, but like the pellucid Charybdea in the clear
water of the harbor, their active movements gave away their presence.
The swimming was very vigorous and was effected by quick, strong
pulsations (as many as 120 per minute were counted), very different from
the slow, rhythmic contractions of the Discomedusan Cassiopea which
was found in the same region over by Port Royal. Whether or not the
animal made intentional efforts to escape capture could not be decided
satisfactorily, but certain it was that they did escape often enough by
swimming quickly below the surface of the semi-opaque water.
Tripedalia endured captivity much more hardily than the Charybdea, and
would live in aquaria happily enough for a number of days--no attempt was
made to see how long. Specimens with their stomach pockets filled with
ripe spermatozoa, or with young at any stage from egg to planula, were
taken in plenty from the latter part of June to the latter part of July.
In each female the young were all at the same stage. The embryos were
thrown out in the aquaria as free-swimming planulæ, which settled down on
the bottom and sides of the glass in a day or two, and quickly developed
into small hydras with mouth and typically with four tentacles (and four
tænioles, W. K. B.), though three and five were by no means uncommon.
In this condition they lived for three weeks without essential change,
and they were still giving no promise of further development when the
laboratory broke up and the jars had to be emptied.
b. _External Anatomy._
The structure of the Cubomedusæ seems to be that of a type well
established, and accordingly offers no very wide range of diversity among
the different genera. The Charybdea that has just been described is a
very typical form and will serve well as a standard with which to compare
our species of Tripedalia. The resemblances are so close that a detailed
account of the anatomy of the second form would involve much needless
repetition. It is hardly necessary to do more than merely point out in
what points Tripedalia resembles Charybdea and in what points it differs.
The form of the bell is less pyramidal than in Charybdea. Some
measurements even gave the breadth greater than the height. The external
surface is divided, as typical for the Cubomedusæ, into the four
perradial sides and the four convex interradial ridges, and the furrows
that separate these areas are with one small exception exactly the same
as those of Charybdea, as may be seen by comparing the series of sections
of Tripedalia (Figs. 21-30) with those of Charybdea (Figs. 6-15). The
exception is almost too slight to mention. The adradial furrow in each
octant which sets off the corner rib from the perradial surface in the
lower part of the bell is not directly continuous, as in Charybdea, with
the corresponding furrow in the upper part of the bell--that is, the
_afr´_ of Figs. 24-27 is not continuous with the _afr_ of Figs. 22 and
23, as is seen by both being shown in Fig. 24. The upper furrow (_afr_)
is continued only a short distance, however, below the starting point of
the lower (_afr´_).
The pedalia conform entirely to the description given those of Charybdea,
except that there are three attached to the bell margin in each
interradius instead of one, and that the blade of each pedalium is much
narrower.
The sensory clubs also show exactly the same relation to the bell and
exactly the same structure.
In the bell cavity the proboscis has a longer and better defined stalk
than that of Charybdea, and has the further and more important difference
of possessing special sensory organs, to the number of fifteen or twenty.
The suspensoria are much more developed than in Charybdea, so that the
interradial funnels lying between are more marked. In a corresponding way
the frenula are larger and stouter (Figs. 28, 29, _frn_). The musculature
shows no new features and differs only in being comparatively more
strongly developed and having a more pronounced striation. The nerve ring
follows the same looped course from the margin in each interradius up to
the level of the sensory clubs in the perradius.
c. _Internal Anatomy._
The stomach offers no peculiarities, and the phacelli also agree with
those of Charybdea except in having a smaller number of filaments in each
tuft. The stomach pockets are not guarded by such well-developed valves
as those described for Charybdea, though the valvular nature of the lips
of the gastric ostia is indicated and the valvular functions undoubtedly
performed. The gastric ostia are smaller (cf. Figs. 7 and 22), and this
makes highly developed valves less necessary. No trace of anything
corresponding to mesogonial pockets was noticed.
In the matter of the marginal pockets, however, we find that the
agreement with Charybdea is no longer continued. The regions that
correspond to the eight marginal pockets of Charybdea are formed, as in
that genus, by the coming together of the exumbrella and subumbrella at
the sensory niche (Figs. 25-28), but each of these regions is subdivided,
as it is not in Charybdea, into two marginal pockets, a larger (_mp_,
Figs. 28-29) and a smaller (_mp´_). In this way sixteen marginal pockets
are formed as in the Chirodropidæ. Furthermore, as happens in the latter
family but does not in the Charybdeidæ, the marginal pockets extend into
the velarium. From each of the larger marginal pockets are given off two
velar canals, while each of the smaller gives rise to but one short one
(Fig. 18). Fig. 30 represents one of the last sections of a Tripedalia
cut transversely, in which nothing but the pedalia and the velarium
appear, and in it are shown the velar canals (_vc_), which come from the
larger marginal pockets. The velarium appears in four segments because it
is drawn upwards in the four perradii by the frenula (see Fig. 20). That
the canals from the smaller pockets do not appear in the section is due
to their shortness and to the fact that they are pulled upwards above the
level of the sections by the frenula, together with that portion of the
velarium.
The smaller velar canals, a pair in each perradius, seem to have in
the males some function in connection with the storing of matured
spermatozoa. In specimens with ripe testes they are very often found
crowded to distension with spermatozoa, while the other velar canals may
or may not contain them, and generally do not. The epithelium lining
them is, like that of the others, composed of columnar cells higher on
the wall turned toward the bell cavity than on that turned towards the
exterior, but otherwise not specially differentiated. I searched in
vain for any trace of opening by which the spermatozoa might gain the
exterior. Fig. 29 shows another point which may be mentioned in passing,
namely, that the canal of each of the three tentacles opens into the
peripheral gastro-vascular system independently. The central tentacle
of each group is the homologue of the single tentacle of Charybdea, and
is formed in Tripedalia before the two lateral tentacles appear. Its
communication with the peripheral pocket system is higher up than the
openings of the lateral tentacles, so that in the section drawn the
latter are just beginning to be indicated (_ct´_).
It remains only to speak of the reproductive organs of Tripedalia. The
sexes are separate in this form also, and ovaries and testes have the
same structure as is found in other Cubomedusæ. The development of
floating masses of cells in the females, however, is a feature which, so
far as I know, has not been observed before. These masses, of which a
small one is represented in section by Fig. 71, are apparently developed
along with the eggs, and repeat the structure of the ovary to all intents
the same as if they were various-sized fragments of it broken loose.
They consist mostly of high, columnar epithelial cells surrounding a
few central cells and showing here and there a nettle cell just as the
reproductive organ does. The epithelial cells differ from those of the
ovary in containing one or more large vacuoles, and this vacuolation
increases as the embryos, among which the masses float, develop. The idea
naturally suggests itself, therefore, that they serve for nourishing
and perhaps for protecting the embryos while they are developing in the
stomach pockets of the mother individual.