the Circe (Fig. 53), differing greatly in outline from the ordinary
Jelly-fishes. As may be seen in Figure 53, the bell forms but a small
portion of the animal; it rises in a sharp cone on the summit,
thinning out at the lower edge, to form the large cavity in which
hangs the long proboscis and the eight ovaries, four of which may be
seen in Fig. 53 crowded with eggs. The Circe differs in consistency,
as well as in form, from other Jelly-fishes. It is hard and horny to
the touch, and the veil, usually so light and filmy, is here a thick
folded membrane, which at every stroke of the animal forces the water
in and out of the cavity. It is very active, moving by powerful jerks,
each one of which throws it far on its way. It advances usually in
straight lines; or, if it wishes to change its direction, it drives
the water out of the veil suddenly on one side or the other, so as to
shoot off, sometimes at right angles with its former path. Four large
pedunculated eyes, hidden in the figure by the tentacles, stand out
prominently from the circular tube. When the animal is in motion, the
tentacles are carried closely curled around the edge of the disk, as
in Fig. 53, where the Circe is represented under a magnifying power of
two and a half diameters. This Jelly-fish is of a delicate rose color,
the tentacles assuming, however, a dark-purple tint at their
extremities when contracted.
_Lucernaria_. (_Haliclystus auricula_ CLARK.)
[Illustration: Fig. 54. Group of Lucernaria attached to eel-grass;
natural size.]
[Illustration: Fig. 55. Lucernaria seen from the mouth side.]
One of the prettiest and most graceful, as well as one of the most
common of our Jelly-fishes, is the Lucernaria (Fig. 54). It has such
an extraordinary contractility of all its parts, that it is not easy
to describe it under any definite form or position, since both are
constantly changing; but perhaps of all its various attitudes and
outlines none are more normal to it than those given in Fig. 54. It
frequently raises itself in the upright position represented here by
the individual highest on the stem, spreading itself in the form of a
perfectly symmetrical cup or vase, the margin of which is indented by
a succession of inverted scallops, the point of junction between every
two scallops being crowned by a tuft of tentacles. But watch it for a
while, and the sides of this vase turn backward, spreading completely
open, till they present the whole inner surface, with the edges even
curved a little downward, drooping slightly, and the proboscis rising
in the centre. In such an attitude one may trace with ease the shape
of the mouth, the lobes surrounding it, as well as the tubes and
cavities radiating from it toward the margin. A touch is, however,
sufficient to make it close upon itself, shrinking together in the
attitude of the third individual in Fig. 54, or even drawing its
tentacles completely in, and contracting all its parts till it looks
like a little ball hanging on the stem. These are but a few of its
manifold changes, for it may be seen in every phase of expansion and
contraction. Let us now look for a moment at the details of its
structure. The resemblance to a cup or vase, as in the upper figure of
the wood-cut (Fig. 54), is deceptive; for a vase is hollow, whereas
the Lucernaria, though so delicate and transparent that its upper
surface, when thus stretched, seems like a mere film, is nevertheless
a solid gelatinous mass, traversed by certain channels, cavities, and
partitions, but otherwise continuous throughout. The peduncle by which
it is attached is but an extension of the floor of a gelatinous disk,
corresponding to that of any Jelly-fish. Four tubes pass through the
whole length of this peduncle, and open into four chambers, dividing
the digestive cavity above into as many equal spaces. (Fig. 55.) These
spaces are produced by folds in the upper floor of the disk, uniting
it to the lower floor at given distances, and forming so many
partition-walls, dividing the digestive sac into four distinct
cavities. These lines of juncture between the two floors, where the
partitions occur, produce the four radiating lines, running from the
proboscis to the margin of the disk, on the upper surface. (Fig. 55.)
The triangular figures, running from the mouth to each cluster of
tentacles, are produced by the ovaries, which consist of distinct
pouches or bags attached to the upper surface of the disk, and hanging
down into the cavities below; every little dot within these triangular
spaces represents such a bag. Each bag is crowded with eggs, which
drop into the digestive cavity at the spawning season, and are passed
out at the mouth. The tentacles always grow in clusters, but are
nevertheless arranged according to a regular order. They are
club-shaped at their extremities, but are hollow throughout, opening
into the chambers of the digestive cavity, two of the clusters thus
being connected with each chamber. Their chief office seems to be to
catch the food and convey it to the mouth, though they may also be
used as a kind of suckers, and the animal not unfrequently attaches
itself by means of these appendages. Between every two clusters of
tentacles will be observed a short, single appendage, of an entirely
different appearance. These are the so-called auricles, and though so
unlike tentacles in the adult animal, when in their earlier stages
(Fig. 56) they resemble each other closely. But as their development
goes on, the tentacles stretch out into longer, more delicate flexible
organs, while the auricles remain short and compact throughout life.
They contain a slight pigment spot representing an eye, though how far
it serves the purpose of vision remains doubtful. They are chiefly
used by the animal as a means of adhering to any surface upon which it
may wish to fasten itself; for the Lucernaria, though usually found
attached to eel-grass in shoal water, has the power of independent
motion, and frequently separates from its resting-place, floating
about freely in the water for a while, or attaching itself anew by
means of the auricles and tentacles upon some other object. The color
of this pretty Acaleph varies from a greenish hue to green, with a
faint tinge of red, or to a reddish brown. One of its commonest and
most exquisite tints is that of a pale aqua-marine. It may be found
along our shores wherever the eel-grass grows, and as far out as this
plant extends. It thrives admirably in confinement, and for this
reason is especially adapted to the aquarium.
[Illustration: Fig. 56. Young Lucernaria; magnified.]
* * * * *
HYDROIDS.
Under this order, the general character of which has already been
explained in the introductory chapter on Acalephs, are included a
number of groups which, whether as Hydroid communities in their
earlier phases of existence, or as free swimming Medusæ in their
farther development, challenge our admiration, both for their beauty
of form and color, and their grace of motion. Some of them are so
minute that they escape the observation of all but those who are
laboriously seeking for the hidden treasures of the microscopic world,
but the greater number are large enough to be readily found by the
most inexperienced collector, when his attention is once drawn to
them; and he may easily stock his aquarium with these pretty little
communities, and even trace the development of the Jelly-fishes upon
them.
To the Hydroids belong the Campanularians, the Sertularians, and the
Tubularians. Some examples of each, as represented on our shores, will
be found under their different heads, accompanied with full
descriptions. There is another group usually considered as distinct
from Hydroids, and known as a separate order among Acalephs, under the
name of Siphonophoræ, but included with them here in accordance with
the views of Vogt, Agassiz, and others, in whose opinion they differ
from the ordinary Hydroid communities only in being free and floating,
instead of fixed to the ground. Some new facts, published here for the
first time, tend to sustain the accuracy of this classification.[5]
With these few preliminary remarks to show the connection of the
order, let us now look at some of the animals belonging to it more in
detail.
[Footnote 5: See Chapter on Nanomia.]
_Campanularians_.
All the Campanularians, of which Oceania (Fig. 68), Clytia (Fig. 73),
and Eucope (Fig. 61) form a part, belong among those little shrub-like
communities of animals called Hydroids, from which most of our
Jelly-fishes are developed. They differ in one essential feature from
the Tubularians. (Fig. 93.) The whole stem, from summit to base, is
enveloped in a horny sheath, extending around both the fertile and
sterile individuals of the community, and forming a network at the
base of the stem, which serves as a kind of foundation for the whole
stock. To the naked eye such a community looks like a tiny shrub (see
Fig. 57), with the branches growing in regular alternation on either
side of the stems. The reproductive calycles, i.e. the protecting
envelopes covering the young Medusæ, usually arise in the angles of
the branches formed by a prolongation of the sheath. These calycles or
bells, as they are called, assume a great variety of
shapes,--elliptical, round, pear-shaped, or ringed like the Clytia.
(Fig. 72.) In one such bell there may be no less than twenty or thirty
Medusæ developed one below the other; when ready to hatch, the calycle
bursts and allows them to escape.
_Eucope_. (_Eucope diaphana_ AG.)
[Illustration: Fig. 57. Hydrarium of Eucope; natural size.]
[Illustration: Fig. 58. Portion of Fig. 57; magnified.]
In Figs. 60 and 61 we have a representation of our little Eucope, one
of the prettiest of the Jelly-fishes belonging to this group; Fig. 57
represents the Hydroid from which it arises; a single branch with the
reproductive bell being magnified in Fig. 58. In Fig. 59 is seen a
portion of the Jelly-fish disk, with the fringe of tentacles highly
magnified. The disk of the Eucope (Fig. 60) looks like a shallow bell,
of which the proboscis often seems to form the handle; for the disk
has such an extraordinary thinness that it turns inside out with the
greatest ease, so that the inner surface may become at any moment the
outer one, with the proboscis projecting from it, as in Fig. 60, while
the next movement of the animal may reverse its whole position, and
the proboscis then hangs down from the inside, as in other
Jelly-fishes. (See Fig. 61.)
[Illustration: Fig. 59. Part of marginal tube and tentacles of
Eucope, greatly magnified; _e_ eye-speck, _b_ base of tentacle.,
_r_ reentering base of tentacle.]
[Illustration: Fig. 60. Young Eucope; magnified.]
[Illustration: Fig. 61. Adult Eucope seen in profile; magnified.]
[Illustration: Fig. 62. Quarter disk of Fig. 60, seen from below;
_e e_ tentacles bearing eye-speck.]
The tentacles are solid and stiff like little hairs, and two of them,
in each quarter-segment of the disk, have small concretions at the
base, which are no doubt eye-specks. (See Fig. 62.) Along the
chymiferous tubes little swellings are developed, which increase
gradually, and become either ovaries or spermaries, according to the
sex of the animal. (Fig. 63.) In the adult the genital organs hang
down, like elongated bags, from the chymiferous tubes. (Fig. 64.) The
tentacles are numerous, multiplying to about a hundred and ninety-two
in the adult, and increasing according to the numerical law to be
explained in the description of the Oceania.
[Illustration: Fig. 63. Quarter-disk of young Eucope, older than
Fig. 62, with a second set of tentacles (2) between the first set
(1).]
[Illustration: Fig. 64. Magnified quarter-disk of adult Eucope.]
This little Jelly-fish is one of the most common in our Bay. There is
not a night or day when they cannot be taken in large numbers, from
the early spring till late in the autumn; and as the breeding season
lasts during the whole of that period, they are found in all possible
stages of growth. In consequence of this, the course of their
development, and the relation between the different phases of their
existence as Hydroids, and afterwards as Acalephs, are well known,
though the successive steps of their growth have not been traced
connectedly, as in some of the other Jelly-fishes, the Tima or
Melicertum, for instance. The process is, however, so similar
throughout the class of Hydroids, that, having followed it from
beginning to end in some of the groups, we have the key to the history
of others, whose development has not been so fully traced. The eggs
laid by the Eucope in the autumn develop into planulæ, which acquire
their full size as Hydroid communities toward the close of the winter,
and the development of the young Medusæ upon them, as described above,
begins with the opening spring.
_Oceania_. (_Oceania languida_ A. AG.)
The Oceania (Fig. 68) is so delicate and unsubstantial, that with the
naked eye one perceives it only by the more prominent outlines of its
structure. We may see the outline of the disk, but not the disk
itself; we may trace the four faint thread-like lines produced by the
radiating tubes traversing the disk from the summit to the margin; and
we may perceive, with far more distinctness, the four ovaries attached
to these tubes near their base; we may see also the circular tube
uniting the radiating tubes, and the tentacles hanging from it, and we
can detect the edge of the filmy veil that fringes the margin of the
disk. But the substance connecting all these organs is not to be
distinguished from the element in which it floats, and the whole
structure looks like a slight web of threads in the water, without our
being able to discern by what means they are held together. Under the
microscope, however, the invisible presently becomes visible, and we
find that this Jelly-fish, like all others, has a solid gelatinous
disk.
[Illustration: Fig. 65. Young Oceania just escaped from its
reproductive calycle; magnified.]
[Illustration: Fig. 66. The same as Fig. 65, from below, still
more magnified; _t_ long tentacles., _t'_ rudimentary tentacle,
_e_ eye-speck on each side of base of tentacles.]
[Illustration: Fig. 67. Young Oceania, older than Fig. 65;
magnified.]
[Illustration: Diagram of succession of tentacles.]
Let us begin with its earlier condition. When it first escapes from
the parent Hydroid stock, the Oceania is almost spherical in form.
(See Fig. 65.) The disk is divided by four chymiferous tubes, running
from the summit to the margin, where they meet the circular tube in
which they all unite. At this time, it has but two well-developed
tentacles, opposite each other on the margin of the disk, just at the
base of two of the chymiferous tubes (Fig. 66), while two others are
just discernible in a rudimentary state, forming slight projections at
the base of the two other tubes. Fig. 66 gives a view of the animal
from below, at this stage of its growth, while Fig. 65 shows it in
profile. It will be seen by the latter how very spherical is the
outline of the disk at this period, while the proboscis, in which are
placed the mouth and digestive cavity, is quite long, and hangs down
considerably below the lower surface of the disk. As the animal
advances in age the disk loses its spherical outline, and becomes much
flattened, as may be seen in Fig. 67. It may be well to introduce here
some explanation of the law according to which the different sets of
tentacles follow each other in successive cycles of growth, since it
is a law of almost universal application in Jelly-fishes and Polyps;
and, owing to the smaller number and simpler arrangement of the
tentacles in Oceania, it may be more easily analyzed in them than in
many others, where the number and complication of the different sets
of tentacles make it very difficult to trace their relation to each
other during their successive growth. We have seen that the Oceania
begins life with only two tentacles. These form the first set, and are
marked with the number 1 in the subjoined diagram, which gives the
plan of all the different sets in their regular order. The second set,
marked 2, consists also of two, which are developed at equal distances
between the first two, i.e. at right angles with them. The third set,
however, marked 3, consists of four, as do all the succeeding sets,
and they are developed between the first and second. The fourth set
comes in between the first and third; the fifth between the third and
second; the sixth between the first and fourth; the seventh between
the fifth and second; the eighth between the third and fourth; the
ninth between the fifth and third. The ultimate number of tentacles in
the Oceania is thirty-two, or sometimes thirty-six, and the cycles
always in twos or multiples of two. But whatever be the number
included in the successive sets of tentacles, and the unit for the
first set ranges from two to forty-eight, the law in different kinds
of Jelly-fishes is always the same, the youngest set always forming
between the oldest preceding set. Thus the fourth set comes in between
the first and third, and the fifth between the second and third, the
intervals occupied now by the fourth set, being limited by the first
set of tentacles on one side, and by the third set on the other side,
while the intervals occupied by the fifth set are bounded by the
second and third sets.
[Illustration: Fig. 68. Adult Oceania; natural size.]
The little spheres represented between the tentacles on the margin of
the disk, in Figs. 65-67, are eye-specks, and these continue to
increase in number with age; in this the Oceania differs from the
Eucope, in which it will be remembered there were but two eye-specks
in each quarter-segment of the disk throughout life. Fig. 68
represents the adult Oceania in full size, when it averages from an
inch and a half to two inches in diameter. It is slow and languid in
its movements, coming to the surface only in the hottest hours of the
summer days; at such times it basks in the sun, turning lazily about,
and dragging its tentacles after it with seeming effort. Sometimes it
remains for hours suspended in the water, not moving even its
tentacles, and offering a striking contrast to its former great
activity when young, and to the lively little Eucope, which darts
through the water at full speed, hardly stopping to rest for a moment.
If the Oceania be disturbed it flattens its disk, and folds itself up
somewhat in the shape of a bale (see Fig. 69), remaining perfectly
still, with the tentacles stretching in every direction. When the
cause of alarm is removed, it gently expands again, resuming its
natural outline and indolent attitudes. The number of these animals is
amazing. At certain seasons, when the weather is favorable, the
surface of the sea may be covered with them, for several miles, so
thickly that their disks touch each other. Thus they remain packed
together in a dense mass, allowing themselves to be gently drifted
along by the tide till the sun loses its intensity, when they retire
to deeper waters. Some points, not yet observed, are still wanting to
complete the history of this Jelly-fish. By comparing such facts,
however, as are already collected respecting it, with our fuller
knowledge of the same process of growth in the Eucope, Tima, and
Melicertum, we may form a tolerably correct idea of its development.
It is hatched from a Campanularia.
[Illustration: Fig. 69. Attitude assumed by Oceania when
disturbed.]
_Clytia_. (_Clytia bicophora_ AG.)
In Figs. 70-73 we have the Acalephian and Hydroid stages of the Clytia
(Fig. 73), another very pretty little Jelly-fish, closely allied to
the Oceania. When first hatched, like the Oceania, it is very convex,
almost thimble-shaped (see Fig. 70), but a little later the disk
flattens and becomes more open, as in Fig. 71. In Fig. 72, we have a
branch of the Hydroid, a Campanularia, greatly magnified, with the
annulated reproductive calycle attached to it, and crowded with
Jelly-fishes ready to make their escape as soon as the calycle bursts.
The adult Clytia (Fig. 73) is somewhat smaller and more active than
the Oceania, and is easily recognized by the black base of its
tentacles, at their point of juncture with the margin of the disk. It
is more commonly found at night, than in the day-time, being nocturnal
in its habits.
[Illustration: Fig. 70. Young Clytia just escaped from the
reproductive calycle.]
[Illustration: Fig. 71. Clytia somewhat older than Fig. 70.]
[Illustration: Fig. 72. Magnified portion of Hydrarium of Clytia.]
[Illustration: Fig. 73. Adult Clytia; twice natural size.]
_Zygodactyla_. (_Zygodactyla groenlandica_ AG.)
Little has been known, and still less published, of this remarkable
genus of Jelly-fish (Figs. 74, 75) up to the present time. The name
Zygodactyla, or Twinfinger, was given to it by Brandt, from drawings
made by Mertens, who had some opportunity of studying it in his
journey around the world. These drawings were published in the
Transactions of the St. Petersburg Academy. In the year 1848 Professor
Agassiz read a paper upon one of the species of this genus belonging
to our coast, before the American Academy, in which he called it
Rhacostoma, not being aware that it had already received a name, and
gave some account of its extraordinary phosphorescent properties. The
name Rhacostoma must of course yield to that of Zygodactyla, which has
a prior claim.
[Illustration: Fig. 74. Zygodactyla seen from above.]
The average size of this Jelly-fish when full grown is from seven to
eight inches in diameter; sometimes it may measure even ten or eleven,
but this is rather rare. The light-violet colored disk is exceedingly
delicate and transparent, its edge being fringed with long fibrous
tentacles, tinged with darker violet at their point of juncture with
the disk, and hanging down a yard and more when fully extended, though
they vary in length according to the size of the specimen, and, in
consequence of their contractile power, may seem much shorter at some
moments than at others. The radiating tubes in this Jelly-fish are
exceedingly numerous, the whole inner surface of the disk being ribbed
with them. (See Figs. 74 and 75.) The ovaries follow the length of the
tubes, though they do not extend quite to their extremity, where they
join the circular tube around the margin of the disk; nor do they
start exactly at the point where the tubes diverge from the central
cavity, but a little below it. (Fig. 74.) Each ovary consists of a
long, brownish, flat bag, split along the middle, so closely folded
together that it seems like a flat blade attached along the length of
the tube. Perhaps a better comparison would be to a pea-pod greatly
elongated, with the edges split along their line of juncture, and
attached to a tube of the same length. The ovaries are not perfectly
straight, but slightly waving, as may be seen in Fig. 74, and these
undulations are stronger when the ovaries are crowded with eggs, as is
the case at the time of spawning.
[Illustration: Fig. 75. Zygodactyla seen in profile.]
The large digestive cavity hangs from the centre of the under side of
the disk (Fig. 75), terminating in the proboscis, which, in this kind
of Jelly-fish, is short in proportion to the diameter of the disk,
while the opening of the mouth is very large. (Fig. 74.) It is
unfortunate that a variety of inappropriate names, likely to mislead
rather than aid the unscientific observer, have been applied to
different parts of the Jelly-fish. What we call here digestive cavity,
proboscis, and mouth, are, in fact, parts of one organ. An exceedingly
delicate, transparent, filmy membrane hangs from the under side of the
disk; that membrane forms the outer wall of the digestive cavity,
which it encloses; it narrows toward its lower margin, leaving open
the circular aperture called the mouth; this narrowing of the membrane
is produced by a number of folds in its lower part, while at its
margin these folds spread out to form ruffles around the edge of the
mouth, and these ruffles again extend into the long scalloped fringes
hanging down below.