Halcyonium (Fig. 22). In the Halcyonoids, the highest group of Polyps,
the tentacles reach their greatest limitation, which, as above
mentioned, is found to be a mark of superiority, and, connected with
other structural features, places them at the head of their class. The
number of tentacles throughout this group is always eight. They are
very complicated (Fig. 21), in comparison with the tentacles of the
lower orders, being deeply lobed, and fringed around the margin. Our
Halcyonium communities (Fig. 22) usually live in deep water, attached
to dead shells, though they may occasionally be found growing at
low-water mark, but this is very rare. They have received a rather
lugubrious name from the fishermen, who call them "dead-men's
fingers," and indeed, when the animals are contracted, such a
community, with its short branches attached to the main stock, looks
not unlike the stump of a hand, with short, fat fingers. In such a
condition they are very ugly, the whole mass being somewhat gelatinous
in texture, and a dull, yellowish pink in color. But when the animals,
which are capable of great extension, are fully spread, as in Fig. 22,
such a polyp-stock has a mossy, tufted look, and is by no means an
unsightly object. When the individuals are entirely expanded, as in
Fig. 23, they become quite transparent, and their internal structure
can readily be seen through the walls of the body; we can then easily
distinguish the digestive cavity, supported for its whole length by
the eight radiating partitions, as well as the great size of the main
digestive cavity surrounding it. Notwithstanding the remarkable power
of contraction and dilatation in the animals themselves, the tentacles
are but slightly contractile. This kind of community increases
altogether by budding, the individual polyps remaining more or less
united, the tissues of the individuals becoming thicker by the
deposition of lime nodules, and thus forming a massive
semi-cartilaginous pulp, uniting the whole community. In the
neighborhood of Provincetown they are very plentiful, and are found
all along the shores of our Bay in deep water.
[Illustration: Fig. 22. Halcyonium community; natural size.]
[Illustration: Fig. 23. Individual of Halcyonium fully expanded;
magnified.]
GENERAL SKETCH OF ACALEPHS.
In the whole history of metamorphosis, that wonderful chapter in the
life of animals, there is nothing more strange or more interesting
than the transformations of the Acalephs. First, as little floating
planulæ or transparent spheres, covered with fine vibratile cilia, by
means of which they move with great rapidity, then as communities
fixed to the ground and increasing by budding like the corals, or
multiplying by self-division, and later as free-swimming Jelly-fishes,
many of them pass through phases which have long baffled the
investigations of naturalists, and have only recently been understood
in their true connection. Great progress has, however, been made
during this century in our knowledge of this class. Thanks to the
investigations of Sars, Dujardin, Steenstrup, Van Beneden, and many
others, we now have the key to their true relations, and transient
phases of growth, long believed to be the adult condition of distinct
animals, are recognized as parts in a cycle of development belonging
to one and the same life. As the class now stands, it includes three
orders, highest among which are the CTENOPHORÆ, so called on account
of their locomotive organs, consisting of minute flappers arranged in
vertical comb-like rows; next to these are the DISCOPHORÆ, with their
large gelatinous umbrella-like disks, commonly called Jelly-fishes,
Sun-fishes, or Sea-blubbers, and below these come the HYDROIDS,
embracing the most minute and most diversified of all these animals.
These orders are distinguished not only by their striking external
differences, but by their mode of development also. The Ctenophoræ
grow from eggs by a direct continuous process of development, without
undergoing any striking metamorphosis; the Discophoræ, with some few
exceptions, in which they develop like the Ctenophoræ from eggs, begin
life as a Hydra-like animal, the subsequent self-division of which
gives rise, by a singular process, presently to be described, to a
number of distinct Jelly-fishes; the Hydroids include all those
Acalephs which either pass the earlier stages of their existence as
little shrub-like communities, or remain in that condition through
life. These Hydroid stocks, as they are sometimes called, give rise to
buds; these buds are transformed into Jelly-fishes, which in some
instances break off when mature and swim away as free animals, while
in others they remain permanent members of the Hydroid stock, never
assuming a free mode of life. All these buds when mature, whether free
or fixed, lay eggs in their turn, from which a fresh stock arises to
renew this singular cycle of growth, known among naturalists as
"alternate generations."
[Illustration: Fig. 24. Branch of Millepora alcicornis; natural
size. (_Agassiz_.)]
[Illustration: Fig. 25. Animals of M. alcicornis expanded;
magnified. _a_ _a_ _a_ small Hydroid, _b_ larger Hydroid, _t_
tentacles, _m_ mouth. (_Agassiz_.)]
The Hydroids are not all attached to the ground,--some like the
Physalia (Portuguese man-of-war), or the Nanomia, that pretty floating
Hydroid of our own waters, move about with as much freedom as if they
enjoyed an individual independent existence. As all these orders have
their representatives on our coast, to be described hereafter in
detail, we need only allude here to their characteristic features. But
we must not leave unnoticed one very remarkable Hydroid Acaleph (Fig.
24), not found in our waters, and resembling the Polyps so much, that
it has long been associated with them. The Millepore is a coral, and
was therefore the more easily confounded with the Polyps, so large a
proportion of which build coral stocks; but a more minute
investigation of its structure (Figs. 25, 26) has recently shown that
it belongs with the Acalephs.[2] This discovery is the more important,
not only as explaining the true position of this animal in the Animal
Kingdom, but as proving also the presence of Acalephs in the earliest
periods of creation, since it refers a large number of fossil corals,
whose affinities with the millepores are well understood, to that
class, instead of to the class of Polyps with which they had hitherto
been associated. But for this we should have no positive evidence of
the existence of Acalephs in early geological periods, the gelatinous
texture of the ordinary Jelly-fishes making their preservation almost
impossible. It is not strange that the true nature of this animal
should have remained so long unexplained; for it is only by the soft
parts of the body, not of course preserved in the fossil condition,
that their relations to the Acalephs may be detected; and they are so
shy of approach, drawing their tentacles and the upper part of the
body into their limestone frame if disturbed, that it is not easy to
examine the living animal.
[Footnote 2: See "Methods of Study," by Prof. Agassiz.]
[Illustration: Fig. 26. Transverse section of a branch, showing
pits, _a_ _a_ _a_ _a_, of the large Hydroids with the horizontal
floors. (_Agassiz_.)]
The Millepore is very abundant on the Florida reefs. From the solid
base of the coral stock arise broad ridges, branching more or less
along the edges, the whole surface being covered by innumerable pores,
from which the diminutive animals project when expanded. (Fig. 25.)
The whole mass of the coral is porous, and the cavities occupied by
the Hydræ are sunk perpendicularly to the surface within the stock.
Seen in a transverse cut these tubular cavities are divided at
intervals by horizontal partitions (Fig. 26), extending straight
across the cavity from wall to wall, and closing it up entirely, the
animal occupying only the outer-most open space, and building a new
partition behind it as it rises in the process of growth. This
structure is totally different from that of the Madrepores, Astræans,
Porites, and indeed, from all the polyp corals which, like all Polyps,
have the vertical partitions running through the whole length of the
body, and more or less open from top to bottom.
The life of the Jelly-fishes, with the exception of the Millepores and
the like, is short in comparison to that of other Radiates. While
Polyps live for many years, and Star-fishes and Sea-urchins require
ten or fifteen years to attain their full size, the short existence of
the Acaleph, with all its changes, is accomplished in one year. The
breeding season being in the autumn, the egg grows into a Hydroid
during the winter; in the spring the Jelly-fish is freed from the
Hydroid stock, or developed upon it as the case may be; it attains its
full size in the fall, lays its eggs and dies, and the cycle is
complete. The autumn storms make fearful havoc among them, swarms of
them being killed by the fall rains, after which they may be found
thrown up on the beaches in great numbers. When we consider the size
of these Jelly-fishes, their rapidity of growth seems very remarkable.
Our common Aurelia measures some twelve to eighteen inches in diameter
when full grown, and yet in the winter it is a Hydra so small as
almost to escape notice. Still more striking is the rapid increase of
our Cyanea, that giant among Jelly-fishes, which, were it not for the
soft, gelatinous consistency of its body, would be one of the most
formidable among our marine animals.
Before entering upon the descriptions of the special kinds of
Jelly-fishes, we would remind our readers that the radiate plan of
structure is reproduced in this class of animals as distinctly as in
the Polyps, though under a different aspect. Here also we find that
there is a central digestive cavity from which all the radiating
cavities, whether simple or ramified, diverge toward the periphery. It
is true that the open chambers of the Polyps are here transformed into
narrow tubes, by the thickening of the dividing partitions; or in
other words, the open spaces of the Polyps correspond to tubes in the
Acalephs, while the partitions in the Polyps correspond to the thick
masses of the body dividing the tubes in the Acalephs. But the
principle of radiation on which the whole branch of Radiates is
constructed controls the organization of Acalephs no less than that of
the other classes, so that a transverse section across any Polyp (Fig.
1), or across any Acaleph (Fig. 50), or across any Echinoderm (Fig.
140), shows their internal structure to be based upon a radiation of
all parts from the centre to the periphery.
That there may be no vagueness as to the terms used hereafter, we
would add one word respecting the nomenclature of this class, whose
aliases might baffle the sagacity of a police detective. The names
Acalephs, Medusæ, or the more common appellation of Jelly-fishes,
cover the same ground, and are applied indiscriminately to the animals
they represent. The name Jelly-fish is an inappropriate one, though
the gelatinous consistency of these animals is accurately enough
expressed by it; but they have no more structural relation to a fish
than to a bird or an insect. They have, however, received this name
before the structure of animals was understood, when all animals
inhabiting the waters were indiscriminately called fishes, and it is
now in such general use that it would be difficult to change it. The
name Medusa is derived from their long tentacular appendages,
sometimes wound up in a close coil, sometimes thrown out to a great
distance, sometimes but half unfolded, and aptly enough compared to
the snaky locks of Medusa. Their third and oldest appellation, that of
Acalephs,--alluding to their stinging or nettling property, and given
to them and like animals by Aristotle, in the first instance, but
afterwards applied by Cuvier in a more limited sense to
Jelly-fishes,--is the most generally accepted, and perhaps the most
appropriate of all.
The subject of nomenclature is not altogether so dry and arid as it
seems to many who do not fully understand the significance of
scientific names. Not only do they often express with terse precision
the character of the animal or plant they signify, but there is also
no little sentiment concealed under these jaw-breaking appellations.
As seafaring men call their vessels after friends or sweethearts, or
commemorate in this way some impressive event, or some object of their
reverence, so have naturalists, under their fabrication of appropriate
names, veiled many a graceful allusion, either to the great leaders of
our science, or to some more intimate personal affection. The _Linnæa
borealis_ was well named after his famous master, by a disciple of the
great Norwegian naturalist; _Goethea semperflorens_, the
ever-blooming, is another tribute of the same kind, while the pretty,
graceful little Lizzia, named by Forbes, is one instance among many of
a more affectionate reference to nearer friends. The allusions of this
kind are not always of so amiable a character, however,--witness the
"Buffonia," a low, noxious weed, growing in marshy places, and named
by Linnæus after Buffon, whom he bitterly hated. Indeed, there is a
world of meaning hidden under our zoölogical and botanical
nomenclature, known only to those who are intimately acquainted with
the annals of scientific life in its social as well as its
professional aspect.
* * * * *
CTENOPHORÆ.
The Ctenophoræ differ from other Jelly-fishes in their mode of
locomotion. All the Discophorous Medusæ, as well as Hydroids, move by
a rhythmical rise and fall of the disk, contracting and expanding with
alternations so regular, that it reminds one of the action of the
lungs, and seems at first sight to be a kind of respiration in which
water takes the place of air. The Greeks recognized this peculiar
character in their name, for they called them Sea-lungs. Indeed,
locomotion, respiration, and circulation are so intimately connected
in all these lower animals, that whatever promotes one of these
functions affects the other also, and though the immediate result of
the contraction and expansion of the disk seems to be to impel them
through the water, yet it is also connected with the introduction of
water into the body, which there becomes assimilated with the food in
the process of digestion, and is circulated throughout all its parts
by means of ramifying tubes. In the Ctenophoræ there is no such
regular expansion and contraction of the disk; they are at once
distinguished from the Discophoræ by the presence of external
locomotive appendages of a very peculiar character. They move by the
rapid flapping of countless little oars or paddles, arranged in
vertical rows along the surface of the disk, acting independently of
each other; one row, or even one paddle, moving singly, or all of them
together, at the will of the animal; thus enabling it to accelerate or
slacken its movements, to dart through the water rapidly, or to
diminish its speed by partly furling its little sails, or, spreading
them slightly, to poise itself with a faint, quivering movement that
reminds one of the pause of the humming-bird in the air,--something
that is neither positive motion, nor actual rest.[3]
[Footnote 3: The flappers of one side are sometimes in full
activity, while those of the other side are perfectly quiet or
nearly so, thus producing rotatory movements in every direction.]
These locomotive appendages are intimately connected with the
circulating tubes, as we shall see when we examine the structural
details of these animals, so that in them also breathing and moving
are in direct relation to each other. To those unaccustomed to the
comparison of functions in animals, the use of the word breathing, as
applied to the introduction of water into the body, may seem
inappropriate, but it is by the absorption of aerated water that these
lower animals receive that amount of oxygen into the system, as
necessary to the maintenance of life in them, as a greater supply is
to the higher animals. The name of Ctenophoræ or comb-bearers, is
derived from these rows of tiny paddles which have been called combs
by some naturalists, because they are set upon horizontal bands of
muscles, see Fig. 29, reminding one of the base of a comb, while the
fringes are compared to its teeth. These flappers add greatly to the
beauty of these animals, for a variety of brilliant hues is produced
along each row by the decomposition of the rays of light upon them
when in motion. They give off all the prismatic colors, and as the
combs are exceedingly small, so that at first sight one hardly
distinguishes them from the disk itself, the exquisite play of color,
rippling in regular lines over the surface of the animal, seems at
first to have no external cause.
_Pleurobrachia_. (_Pleurobrachia rhododactyla Ag_.)
[Illustration: Fig. 27. Pleurobrachia seen at right angles to the
plane in which the tentacles are placed. (_Agassiz_.)]
Among the most graceful and attractive of these animals are the
Pleurobrachia (Fig. 29), and, though not first in order, we will give
it the precedence in our description, because it will serve to
illustrate some features of the other two groups. The body of the
Pleurobrachia consists of a transparent sphere, varying, however, from
the perfect sphere in being somewhat oblong, and also by a slight
compression on two opposite sides (Figs. 27 and 28), so as to render
its horizontal diameter longer in one direction than in the other
(Fig. 30). This divergence from the globular form, so slight in
Pleurobrachia as to be hardly perceptible to the casual observer,
establishing two diameters of different lengths at right angles with
each other, is equally true of the other genera. It is interesting and
important, as showing the tendency in this highest group of Acalephs
to assume a bilateral character. This bilaterality becomes still more
marked in the highest class of Radiates, the Echinoderms. Such
structural tendencies in the lower animals, hinting at laws to be more
fully developed in the higher forms, are always significant, as
showing the intimate relation between all parts of the plan of
creation. This inequality of the diameters is connected with the
disposition of parts in the whole structure, the locomotive fringes
and the vertical tubes connected with them being arranged in sets of
four on either side of a plane passing through the longer diameter,
showing thus a tendency toward the establishment of a right and left
side of the body, instead of the perfectly equal disposition of parts
around a common centre, as in the lower Radiates.
[Illustration: Fig. 28. Pleurobrachia seen in plane of tentacles.
(_Agassiz_.)]
The Pleurobrachia are so transparent, that, with some preparatory
explanation of their structure, the most unscientific observer may
trace the relation of parts in them. At one end of the sphere is the
transverse split (Fig. 27), that serves them as a mouth; at the
opposite pole is a small circumscribed area, in the centre of which is
a dark eye-speck. The eight rows of locomotive fringes run from pole
to pole, dividing the whole surface of the body like the ribs on a
melon. (Figs. 27, 28.) Hanging from either side of the body, a little
above the area in which the eye-speck is placed, are two most
extraordinary appendages in the shape of long tentacles, possessing
such wonderful power of extension and contraction that, while at one
moment they may be knotted into a little compact mass no bigger than a
pin's head, drawn up close against the side of the body, or hidden
within it, the next instant they may be floating behind it in various
positions to a distance of half a yard and more, putting out at the
same time soft plumy fringes (Fig. 29) along one side, like the beard
of a feather. One who has never seen these animals may well be
pardoned for doubting even the most literal and matter-of-fact account
of these singular tentacles. There is no variety of curve or spiral
that does not seem to be represented in their evolutions. Sometimes
they unfold gradually, creeping out softly and slowly from a state of
contraction, or again the little ball, hardly perceptible against the
side of the body, drops suddenly to the bottom of the tank in which
the animal floating, and one thinks for a moment, so slight is the
thread-like attachment, that it has actually fallen from the body; but
watch a little longer, and all the filaments spread out along the side
of the thread, it expands to its full length and breadth, and resumes
all its graceful evolutions.
[Illustration: Fig. 29. Natural attitude of Pleurobrachia when in
motion.]
[Illustration: Fig. 30. Pleurobrachia seen from the extremity
opposite the mouth.]
One word of the internal structure of these animals, to explain its
relation to the external appendages. The mouth opens into a wide
digestive cavity (Figs. 27, 28), enclosed between two vertical tubes.
Toward the opposite end of the body these tubes terminate or unite in
a single funnel-like canal, which is a reservoir as it were for the
circulating fluid poured into it through an opening in the bottom of
the digestive cavity. The food in the digestive cavity becomes
liquefied by mingling with the water entering with it at the mouth,
and, thus prepared, it passes into this canal, from which, as we shall
presently see, all the circulating tubes ramifying throughout the body
are fed. Two of these circulating tubes, or, as they are called from
the nature of the liquid they contain, chymiferous tubes, are very
large, starting horizontally and at right angles with the digestive
cavity from the point of junction between the vertical tubes (Fig. 30)
and the canal. Presently they give off two branches, those again
ramifying in two directions as they approach the periphery, so that
each one of the first main tubes has multiplied to four, before its
ramifications reach the surface, thus making in all eight radiating
tubes. So far, these eight tubes are horizontal, all diverging on the
same level; but as they reach the periphery each one gives rise to a
vertical tube, running along the surface of the body from pole to
pole, just within the rows of locomotive fringes on the outer surface,
and immediately connected with them (Figs. 27, 28). As in all the
Ctenophoræ, these fringes keep up a constant play of color by their
rapid vibrations. In Pleurobrachia the prevailing tint is a yellowish
pink, though it varies to green, red, and purple, with the changing
motions of the animal. We have seen that the vertical tubes between
which the digestive cavity is enclosed, start like the cavity itself
from that pole of the body where the mouth is placed, and that, as
they approach the opposite pole, at a distance from the mouth of about
two thirds the whole length of the body, they unite in the canal,
which then extends to the other pole where the eye-speck is placed. As
it is just at this point of juncture between the tubes and the canal
that the two main horizontal tubes arise from which all the others
branch on the same plane (Figs. 27, 28), it follows that they reach
the periphery, not on a level with the pole opposite the mouth, but
removed from it by about one third the height of the body. In
consequence of this the eight vertical tubes arising from the
horizontal ones, in order to run the entire length of the body from
pole to pole, extend in opposite directions, sending a branch to each
pole, though the branch running toward the mouth is of course the
longer of the two. The tentacles have their roots in two sacs within
the body, placed at right angles with the split of the mouth. (Figs.
27, 30.) They open at the surface on the opposite side from the mouth,
though not immediately within the area at which the eye-speck is
placed, but somewhat above it, and at a little distance on either side
of it. The tentacles may be drawn completely within these sacs, or be
extended outside, as we have seen, to a greater or less degree, and in
every variety of curve or spiral.
_Bolina_. (_Bolina alata_ AG.)
[Illustration: Fig. 31. Bolina seen from the broad side; _o_
eye-speck, _m_ mouth, _r_ auricles, _v_ digestive cavity, _g_ _h_
short rows of flappers, _a_ _f_ long rows of flappers, _n_ _x_ _t_
_z_ tubes winding in the larger lobes; about half natural size.
(_Agassiz_.)]
[Illustration: Fig. 32. Bolina seen from the narrow side; _c_ _h_
short rows of flappers, _a_ _b_ long rows of flappers; other
letters as in Fig. 31. (_Agassiz_.)]