with a longitudinal split at one end of the body, forming a mouth
which opens into a capacious sac or digestive cavity. But it differs
from the Pleurobrachia in having the oral end of the body split into
two larger lobes (Fig. 31), hanging down from the mouth. These lobes
may gape widely, or they may close completely over the mouth so as to
hide it from view, and their different aspects under various degrees
of expansion or contraction account for the discrepancies in the
description of these animals. We have seen that the Pleurobrachia
moves with the mouth upward; but the Bolina, on the contrary, usually
carries the mouth downward, though it occasionally reverses its
position, and in this attitude, with the lobes spread open, it is
exceedingly graceful in form, and looks like a white flower with the
crown fully expanded. These broad lobes are balanced on the other
sides of the body by four smaller appendages, divided in pairs, two on
each side (Fig. 32), called auricles. These so-called auricles are in
fact organs of the same kind as the larger lobes, though less
developed. The rows of locomotive flappers on the Bolina differ in
length from each other (Fig. 31), instead of being equal, as in the
Pleurobrachia. The four longest ones are opposite each other on those
sides of the body where the larger lobes are developed, the four short
ones being in pairs on the sides where the auricles are placed. At
first sight they all seem to terminate at the margin of the body, but
a closer examination shows that the circulating tubes connected with
the longer row extend into the lobes, where they wind about in a
variety of complicated involutions. (Fig. 32.) The movements of the
Bolina are more sluggish than those of the Pleurobrachia, and the long
tentacles, so graceful an ornament to the latter, are wanting in the
former. With these exceptions the description given above of the
Pleurobrachia will serve equally well for the Bolina. The structure is
the same in all essential points, though it differs in the size and
proportion of certain external features, and its play of color is less
brilliant than that of the Pleurobrachia. The Bolina, with its slow,
undulating motion, its broad lobes sometimes spreading widely, at
other times folded over the mouth, its delicacy of tint and texture,
and its rows of vibrating fringes along the surface, is nevertheless a
very beautiful object, and well rewards the extreme care without which
it dies at once in confinement.
_Idyia_. (_Idyia roseola_ AG.)
[Illustration: Fig. 33. Idyia roseola seen from the broad side,
half natural size; _a_ anal opening, _b_ lateral tube, _c_
circular tube, _d_ _e_ _f_ _g_ _h_ rows of locomotive flappers.
(_Agassiz_.)]
The lowest genus of Ctenophoræ found on our coast, the Idyia (Fig.
33), has neither the tentacles of the Pleurobrachia, nor the lobes of
the Bolina. It is a simple ovate sphere, the interior of which is
almost entirely occupied by an immense digestive cavity. It would seem
that the reception and digestion of food is intended to be the almost
exclusive function of this animal, for it has a mouth whose ample
dimensions correspond with its capacious stomach. Instead of the
longitudinal split serving as a mouth, in the Bolina and
Pleurobrachia, one end of the body in the Idyia is completely open
(Fig. 33), so that occasionally some unsuspicious victim of smaller
diameter than itself may be seen to swim into this wide portal, when
suddenly the door closes behind him with a quick contraction, and he
finds himself a prisoner. The Idyia does not always obtain its food
after this indolent fashion however, for it often attacks a Bolina or
Pleurobrachia as large or even larger than itself, when it extends its
mouth to the utmost, slowly overlapping the prey it is trying to
swallow by frequent and repeated contractions, and even cutting off by
the same process such portions as cannot be forced into the digestive
cavity.
The general internal structure of the Idyia corresponds with that of
the Bolina and Pleurobrachia; it has the same tubes branching
horizontally from the main cavity, then ramifying as they approach the
periphery till they are multiplied to eight in all, each of which
gives off one of the vertical tubes connected with the eight rows of
locomotive flappers. Opposite the mouth is the eye-speck, placed as in
the two other genera, at the centre of a small circumscribed area,
which in the Idyia is surrounded by delicate fringes, forming a
rosette at this end of the body. These animals are exceedingly
brilliant in color; bright pink is their prevailing hue, though pink,
red, yellow, orange, green, and purple, sometimes chase each other in
quick succession along their locomotive fringes. At certain seasons,
when most numerous, they even give a rosy tinge to patches on the
surface of the sea. Their color is brightest and deepest before the
spawning season, but as this advances, and the ovaries and spermaries
are emptied, they grow paler, retaining at last only a faint pink
tint. They appear early in July, rapidly attain their maximum size,
and are most numerous during the first half of August. Toward the end
of August they spawn, and the adults are usually destroyed by the
early September storms, the young disappearing at the same time, not
to be seen again till the next summer. It is an interesting question,
not yet solved, to know what becomes of the summer's brood in the
following winter. They probably sink into deep waters during this
intervening period. The Idyia, like the Pleurobrachia, moves with the
mouth upward, but inclined slightly forward also, so as to give an
oblique direction to the axis of the body.[4]
[Footnote 4: Until this summer only the three genera of Ctenophoræ
above mentioned were supposed to exist along our coast, but during
the present season I have had the good fortune to find two
additional ones. One of them, the Lesueuria, resembles a Bolina
with the long lobes so cut off, that they have a very stunted
appearance in comparison with those of the Bolina. The other, the
Mertensia, is closely allied to Pleurobrachia; it is exceedingly
flattened and pear-shaped. This species was discovered long ago by
Fabricius, but had escaped thus far the attention of other
naturalists. (_A. Agassiz_.)]
EMBRYOLOGY OF CTENOPHORÆ.
All the Ctenophoræ are reproduced from eggs, these eggs being so
transparent that one may follow with comparative ease the changes
undergone by the young while still within the egg envelope.
Unfortunately, however, they are so delicate that it is impossible to
keep them alive for any length of time, even by supplying them
constantly with fresh sea-water, and keeping them continually in
motion, both of which are essential conditions to their existence. It
is therefore only from eggs accidentally fished up at different stages
of growth that we may hope to ascertain any facts respecting the
sequence of their development. When hatched, the little Ctenophore is
already quite advanced. It is small when compared with the size of the
egg envelope, and long before it is set free, it swims about with
great velocity within the walls of its diminutive prison (Fig. 35).
The importance of studying the young stages of animals can hardly find
a better illustration than among the Ctenophoræ. Before their
extraordinary embryonic changes were understood, many of the younger
forms had found their way into our scientific annals as distinct
animals, and our nomenclature thus became burdened with long lists of
names which will disappear as our knowledge advances.
The great size of their locomotive flappers in proportion to the rest
of the body, is characteristic of the young Ctenophoræ. They seem like
large paddles on the sides of these tiny transparent spheres, and,
owing to their great power as compared with those of the adult, the
young move with extraordinary rapidity. The Pleurobrachia alone
retains its quickness of motion in after life, and although its long
graceful streamers appear only as short stumpy tentacles in the young
(Fig. 34), yet its active little body would be more easily recognized
in the earlier stages of growth than that of the other Ctenophoræ.
Figs. 34, 35, and 36 show the Pleurobrachia at various stages of
growth; Fig. 34, with its thick stunted tentacles and short rows of
flappers, is the youngest; the flappers themselves are rather long at
this age, looking more like stiff hairs than like the minute fringes
of the adult. In Fig. 35 the tentacles are already considerably longer
and more delicate; in Fig. 36 the vertical tubes are already
completed, while Figs. 27-29 present it in its adult condition.
[Illustration: Fig. 34. Young Pleurobrachia still in the egg; _t_
tentacle, _e_ eye-speck, _c_ _c_ rows of locomotive flappers, _d_
digestive cavity; greatly magnified.]
[Illustration: Fig. 35. Young Pleurobrachia swimming about in the
egg just before hatching; magnified.]
[Illustration: Fig. 36. Young Pleurobrachia resembling somewhat
the adult; _f_ funnel leading to anal opening, _l_ lateral tubes,
_o_ _o_ _o'_ _o'_ rows of locomotive flappers; magnified.]
[Illustration: Fig. 37. Young Idyia, greatly magnified; lettering
as in Fig. 36; _d_ digestive cavity.]
The Idyia differs greatly in appearance at different periods of its
development, and, indeed, no one would suspect, without some previous
knowledge of its transformations, that the young Idyia, with its rapid
gyrations, its short ambulacral tubes, like immense pouches (Fig. 37),
its large pigment spots scattered over the surface (Fig. 38), was an
earlier stage of the rosy-hued Idyia, which glides through the water
with a scarcely perceptible motion. Figs. 37-40 represent the various
stages of its growth. It will be seen how very short are the
locomotive fringes (Fig. 39) in comparison with those of the
full-grown ones (Fig. 33). It is only in the adult Idyia that these
rows attain their full height, and the tubes, ramifying throughout the
body (Fig. 40), are completed.
[Illustration: Fig. 38. Young Idyia seen from the anal extremity,
magnified; _a_ anal opening, other letters as in Fig. 36.]
[Illustration: Fig. 39. Idyia somewhat older than Fig. 37,
lettering as before; magnified.]
[Illustration: Fig. 40. Young Idyia in which the ambulacral tubes
begin to ramify; magnified, letters as before.]
The Bolina, in its early condition, recalls the young Pleurobrachia.
At this period it has the same rapid motion, and when somewhat more
advanced, long tentacles, resembling those of the Pleurobrachia, make
their appearance (Fig. 41); it is only at a later period that the
tentacles become contracted, while the large lobes (Fig. 42), so
characteristic of Bolina, are formed by the elongation of the oral end
of the body, the auricles becoming more conspicuous at the same time
(Fig. 43). A little later the lobes enlarge, the movements become more
lazy; it assumes both in form and habits the character of the adult
Bolina.
[Illustration: Fig. 41. Young Bolina in stage resembling
Pleurobrachia; greatly magnified.]
[Illustration: Fig. 42. Young Bolina seen from the broad side,
with rudimentary auricles and lobes; magnified.]
[Illustration: Fig. 43. The same as Fig. 42, seen from the narrow
side.]
The series of changes through which the Ctenophoræ pass are as
remarkable as any we shall have occasion to describe, though not
accompanied with such absolutely different conditions of existence.
The comparison of the earlier stages of life in these animals with
their adult condition is important, not only with reference to their
mode of development, but also because it gives us some insight into
the relative standing of the different groups, since it shows us that
certain features, permanent in the lower groups, are transient in the
higher ones. A striking instance of this occurs in the fact mentioned
above, that though the long tentacles so characteristic of the adult
Pleurobrachia exist in the young Bolina, they yield in importance at a
later period to the lobes which eventually become the predominant and
characteristic feature of the latter.
* * * * *
DISCOPHORÆ.
The disk of the Discophoræ is by no means so delicate as that of the
other Jelly-fishes. It seems indeed quite solid, and somewhat like
cartilage to the touch, and yet so large a part of its bulk consists
of water, that a Cyanea, weighing when alive about thirty-four pounds,
being left to dry in the sun for some days, was found to have lost
about 99/100 of its original weight,--only the merest film remaining
on the paper upon which it had been laid. The prominence of the disk
in this group of Jelly-fishes is well characterized by their German
name, "Scheiben quallen," viz. disk-medusæ. We shall see hereafter
that the disk, so large and seemingly solid in the Discophoræ, thins
out in many of the other Jelly-fishes, and becomes exceedingly
concave. This is especially the case in many of the Hydroid Medusæ,
where it assumes a bell-shaped form, and is constantly spoken of as
the bell. It should be remembered, however, in reading descriptions of
these animals, that the so-called bell is only a modified disk, and
perfectly homologous with that organ in the Discophoræ.
The Discophorous Medusæ are distinguished from all others by the
peculiar nature of the reproductive organs. They are contained in
pouches (Fig. 50, _o_, _o_, _o_, _o_), the contents of which are first
discharged into the main cavity, and then pass out through the mouth.
Pillars support the four angles of the digestive cavity, thus
separating the lower from the upper floor of the disk, while the
chymiferous tubes (Fig. 50) branch and run into each other near the
periphery, forming a more or less complicated anastomosing network,
instead of a simple circular tube, as is the case with the Hydroid
Medusæ. (Fig. 74.)
_Cyanea_. (_Cyanea arctica_ PÉR. et LES.)
In our descriptions of the Discophoræ, we may give the precedence to
the Cyanea on account of its size. This giant among Jelly-fishes is
represented in Fig. 44. It is much to be regretted that these animals,
when they are not so small as to escape attention altogether, are
usually seen out of their native element, thrown dead or dying on the
shore, a mass of decaying gelatinous matter. All persons who have
lived near the sea are familiar with the so-called Sea-blubbers,
sometimes strewing the sandy beaches after the autumn storms in such
numbers that it is difficult to avoid them in walking or driving. In
such a condition the Cyanea is far from being an attractive object; to
form an idea of his true appearance, one must meet him as he swims
along at midday, rather lazily withal, his huge semi-transparent disk,
with its flexible lobed margin, glittering in the sun, and his
tentacles floating to a distance of many yards behind him.
Encountering one of those huge Jelly-fishes, when out in a row-boat
one day, we attempted to make a rough measurement of his dimensions
upon the spot. He was lying quietly near the surface, and did not seem
in the least disturbed by the proceeding, but allowed the oar, eight
feet in length, to be laid across the disk, which proved to be about
seven feet in diameter. Backing the boat slowly along the line of the
tentacles, which were floating at their utmost extension behind him,
we then measured these in the same manner, and found them to be rather
more than fourteen times the length of the oar, thus covering a space
of some hundred and twelve feet. This sounds so marvellous that it may
be taken as an exaggeration; but though such an estimate could not of
course be absolutely accurate, yet the facts are rather understated
than overstated in the dimensions here given. And, indeed, the
observation was more careful and precise than the circumstances would
lead one to suppose, for the creature lay as quietly, while his
measure was taken, as if he had intended to give every facility for
the operation. This specimen was, however, of unusual size; they more
commonly measure from three to five feet across the disk, while the
tentacles may be thirty or forty feet long. The tentacles are
exceedingly numerous (see Fig. 44), arising in eight distinct bunches,
from the margin of the disk, and hanging down in a complete labyrinth.
These animals are not so harmless as it would seem, from their soft,
gelatinous consistency; it is no pleasant thing when swimming or
bathing to become entangled in this forest of fine feelers, for they
have a stinging property like nettles, and may render a person almost
insensible, partly from pain, and partly from a numbness produced by
their contact, before he is able to free himself from the network in
which he is caught. The weapons by which they produce these results
seem so insignificant, that one cannot but wonder at their power. The
tentacles are covered by minute cells, lasso-cells as they are called,
(similar to those of Astrangia, Fig. 19,) each one of which contains a
whip finer than the finest thread, coiled in a spiral within it.
[Illustration: Fig. 44. Cyanea arctica; greatly reduced in size.]
These myriad whips can be thrown out at the will of the animal, and
really form an efficient galvanic battery. Behind the veil of
tentacles, and partly hidden by it, four curtains, with lobed or
ruffled margins, dimly seen in Fig. 44, hang down from the under
surface of the disk. The ovaries are formed by four pendent pouches,
placed near the sides of the mouth, and attached to four cavities
within the disk. Around the circumference of the disk are eight
eye-specks, each formed by a small tube protected under a little
lappet or hood rising from the upper surface of the disk. The
prevailing color of this huge Jelly-fish is a dark brownish-red, with
a light, milk-white margin, tinged with blue, the tentacles and other
pendent appendages having a somewhat different hue from the disk. The
ovaries are flesh-colored, the curtain formed by the expansion of the
lobes of the mouth is dark brown, while the tentacles are of different
colors, some being yellow, others purple, and others reddish brown or
pink.
[Illustration: Fig. 45. Scyphistoma of a Discophore; Aurelia
flavidula. (_Agassiz_.)]
[Illustration: Fig. 47. Strobila of a Discophore; Aurelia
flavidula. (_Agassiz_.)]
[Illustration: Fig. 46. Scyphistoma, older than Fig. 45.
(_Agassiz_.)]
Strange to say, this gigantic Discophore is produced by a Hydroid
measuring not more than half an inch in height when full grown; could
we follow the history of any egg laid by one of these Discophoræ in
the autumn, and this has indeed been partially done, we should see
that, like any other planula, the young hatched from the egg is at
first spherical, but presently becomes pear-shaped, and attaches
itself to the ground. From the upper end tentacles project (see Fig.
45), growing more numerous, as in Fig. 46, though they never exceed
sixteen in number. As it increases in height constrictions take place
at different distances along its length, every such constriction being
lobed around its margin, till at last it looks like a pile of
scalloped saucers or disks strung together (see Fig. 47). The topmost
of these disks falls off and dies; but all the others separate by the
deepening of the constrictions, and swim off as little free disks
(Fig. 48), which eventually grow into the enormous Jelly-fish
described above. These three phases of growth, before the relation
between them was understood, have been mistaken for distinct animals,
and described as such under the names of Scyphistoma, Strobila, and
Ephyra.
[Illustration: Fig. 48. Ephyra of a Discophore; Aurelia flavidula.
(_Agassiz_.)]
_Aurelia_. (_Aurelia flavidula_ PÉR. et LES.)
[Illustration: Fig. 49. Aurelia seen in profile, reduced.
(_Agassiz_.)]
Another large Discophore, though by no means to be compared to the
Cyanea in size, is our common Aurelia (Figs. 49, 50) Its bluish-white
disk measures from twelve to fifteen inches in diameter, but its
dimensions are not increased by the tentacles, which have no great
power of contraction and expansion, and form a short fringe around its
margin, widening and narrowing slightly as the tentacles are stretched
or drawn in. It is quite transparent, as may be seen in Fig 49, where
all the fine ramifications of the chymiferous tubes as well as the
ovaries, are seen through the vault of the disk. Fig. 50 represents
the upper surface, with the ovaries around the mouth, occupying the
same position as those of the Cyanea, though they differ from the
latter in their greater rigidity, and do not hang down in the form of
pouches. The males and females in this kind of Jelly-fish may be
distinguished by the difference of color in the reproductive organs,
which are rose-colored in the males, and of a dull yellow in the
females. The process of development is exactly the same in the Aurelia
as in the Cyanea, though there is a very slight difference in their
respective Hydroids. They are, however, so much alike, that one is
here made to serve for both, the above figures being taken from the
Hydroid of the Aurelia. It is curious, that while, as in the case of
the Aurelia and Cyanea, very dissimilar Jelly-fishes may arise from
almost identical Hydroids, we have the reverse of the proposition, in
the fact that Hydroids of an entirely distinct character may produce
similar Jelly-fishes.
[Illustration: Fig. 50. Aurelia flavidula, seen from above; _o_
mouth, _e e e e_ eyes, _m m m m_ lobes of the mouth, _o o o o_
ovaries, _t t t t_ tentacles, _w w_ ramified tubes. (_Agassiz_.)]
The embryos or little planulæ, hatched from the Cyanea and Aurelia in
the fall, seem to be gregarious in their mode of life, swimming about
together in great numbers till they find a suitable point of
attachment, and assume their fixed Hydroid existence. The Cyaneæ,
however, when adult, are usually found singly, while the Aureliæ, on
the contrary, seek each other, and commonly herd together.
_The Campanella_. (_Campanella pachyderma_ A. AG.)
[Illustration: Fig. 51. Campanella seen in profile; greatly
magnified.]
[Illustration: Fig. 52. Same, seen from below.]
The Campanella (Fig. 51) is a pretty little Jelly-fish, not larger
than a pin's head, reproduced directly from eggs, without passing
through the Hydroid stage. During its early stages of growth it
probably remains attached to floating animals, thus leading a kind of
parasitic existence; but as its habits are not accurately known, this
cannot be asserted as a constant fact respecting them. The veil in
this Jelly-fish is very large, forming pendent pouches hanging from
the circular canal (see Fig. 51), and leaving but just room enough for
the passage of the proboscis between the folds. It may not be amiss to
introduce here a general account of this organ, which occurs in many
of the Medusæ, though it has very different proportions in the various
kinds. It is a delicate membrane, hanging from the circular tube, so
as partially to close the mouth of the bell, leaving a larger or
smaller opening for the passage of the water, which is taken in and
forced out again by the alternate expansions and contractions of the
bell.
There are but four chymiferous tubes in the Campanella, and four stiff
tentacles, which in consequence of the peculiar character of the veil
appear, when the animal is seen in profile, to start from the middle
of the disk. The ovaries consist of eight pouches, placed near the
point of junction of the four chymiferous tubes. (Fig. 52.) This
little Medusa is of a dark yellowish color with brownish ocellated
spots, scattered profusely over the upper part of the disk.
_Circe_. (_Trachynema digitale_ A. AG.)
[Illustration: Fig. 53. Trachynema digitale; about twice the
natural size.]