phosphorescence to greatest advantage you must choose a dark night,
when the motion of your boat sets the sea on fire around you, and a
long undulating wave of light rolls off from your oar as you lift it
from the water. On a brilliant evening this effect is lost in a great
degree, and it is not until you dip your net fairly under the moonlit
surface of the sea, that you are aware how full of life it is.
Occasionally one is tempted out by the brilliancy of the
phosphorescence, when the clouds are so thick that water, sky, and
land become one indiscriminate mass of black, and the line of rocks
can be discerned only by the vivid flash of greenish golden light,
when the breakers dash against them. At such times there is something
wild and weird in the whole scene, which at once fascinates and
appalls the imagination; one seems to be rocking above a volcano, for
the surface around is intensely black, except where fitful flashes or
broad waves of light break from the water under the motion of the boat
or the stroke of the oars. It was on a night like this, when the
phosphorescence was unusually brilliant, and the sea as black as ink,
the surf breaking heavily and girdling the rocky shore with a wall of
fire, that our collector was so fortunate as to find in the rich
harvest he brought home the entirely new and exceedingly pretty little
floating Hydroid, described under the name of Nanomia (Fig. 115). It
was in its very infancy (Fig. 108), a mere bubble, not yet possessed
of the various appendages which eventually make up its complex
structure; but it was nevertheless very important to have seen it in
this early stage of its existence, since, when a few full-grown
specimens were found in the autumn, which lived for some days in
confinement and quietly allowed their portraits to be taken (see Fig.
115), it was easy to connect the adult animal with the younger phase
of its own life and thus make a complete history.
Marine phosphorescence is no new topic, and we have dwelt too long,
perhaps, upon a phenomenon that every voyager has seen, and many have
described. Its effect is very different, when seen from the deck of a
vessel, from its appearance as one floats through its midst,
distinguishing the very creatures that produce it, and any account of
the Medusæ which did not include this most characteristic feature
would be incomplete.
* * * * *
ECHINODERMS.
Our illustrations and descriptions of Echinoderms are scanty in
comparison to those of the preceding class; for while, in consequence,
perhaps, of the combined influence of the Gulf Stream and the cold
arctic current on the New England shore, Acalephs are largely
represented in our waters, our marine fauna is meagre in Echinoderms.
But although we have few varieties, those which do establish
themselves on a coast seemingly so ungenial for others of their kind,
such as the Echinus, and our common Star-fish, for instance, thrive
well and are very abundant. The class of Echinoderms includes five
orders, viz. CRINOIDS, OPHIURANS, STAR-FISHES, SEA-URCHINS, and
HOLOTHURIANS. The animals composing these orders differ so widely in
appearance that it was very long before their true relations were
detected, and it was seen that all their external differences were
united under a common plan. Let us compare, for instance, the
worm-like Holothurians (Figs. 124, 126, 127) with all the host of
Star-fishes (Figs. 142, 146, 147) and Sea-urchins (Figs. 131, 139), or
compare the radiating form of the Star-fish, its arms spreading in
every direction, with the close spherical outline of the Sea-urchin,
or the Crinoid floating at the end of a stem (Fig. 152) with either of
these, and we shall cease to wonder that naturalists failed to find at
once a unity of idea under all these varieties of execution. And yet
the fundamental structure of the class of Echinoderms is represented
as distinctly by any one of its five orders as by any other, and is
absolutely identical in all. They differ only by trifling
modifications of development.
In Echinoderms as a class, the body presents three regions differing
in structure, and on the greater or less development of these regions
or systems, as we may call them, their chief differences are based.
Take, for instance, the dorsal system, the nature of which is
explained by the name, indicating of course the back of the animal,
though it does not necessarily imply the upper side of the body, since
some of the Echinoderms, as the stemmed Crinoids, for example, carry
the dorsal side downward, while the Star-fishes and Sea-urchins carry
it upward, and the Holothurians, moving with the mouth forward, have
the dorsal system at the opposite end of the body. Whatever the
natural attitude of the animal, however, and the consequent position
of the dorsal region, it exists alike in all the five orders, though
it has not the same extent and importance in each. But in all it is
made up of similar parts, bears the same relation to the rest of the
body, has the same share in the general economy of the animal. And
though when we compare the spreading back of a Star-fish with the
small area on the top of a Sea-urchin, where all the zones unite, we
may not at once see the correspondence between them, yet a careful
comparison of all their structural details shows that they are both
built with the same elements and represent the same region, though it
is stretched to the utmost in the one case, and greatly contracted in
the other.
This being true of the dorsal system, let us look at another equally
important structural feature in this class. All Echinoderms have
locomotive organs peculiar to themselves, a kind of suckers which may
be more or less numerous, larger or smaller, in different species, but
are always appendages of the same character. These are variously
distributed over the body, but always with a certain regularity
occupying definite spaces, shown by investigation to be homologous in
all. For instance, the rays of the Star-fish correspond in every
detail on their under side, along which the locomotive suckers run,
with the zones on the Sea-urchin, from end to end of which the suckers
are arranged; and the same is equally true of the distribution of the
suckers on the Holothurians, Ophiurans, and Crinoids, though, as most
persons are less familiar with these orders than with the other two,
it might not be so easy to point out the coincidence to our readers.
These suckers are called the ambulacra, the lines along which they run
are called the ambulacral rows or zones, while the system of
locomotion as a whole is known as the ambulacral system. Since these
organs are thus regularly distributed over the body in distinct zones
or rows, it follows that the latter must be divided by intervening
spaces. These intervals are called the interambulacral spaces; but
while in some orders they are occupied by larger plates and prominent
spines, as in the Sea-urchin and Star-fish, in others they are either
comparatively insignificant or completely suppressed, as in the
Crinoids and Ophiurans. Such are the three regions or systems which by
their greater or less development introduce an almost infinite variety
of combinations into this highest class of Radiates. It may not be
amiss before proceeding further to compare the five orders with
reference to this point, and see which of these three systems has the
preponderance in each one.
Taking the orders in their rank and beginning with the lowest, we find
in the Crinoids that the dorsal system preponderates, being composed
of highly complicated plates, and developed to such a degree as to
form in many instances a stem by which the animal is attached to the
ground, while the ambulacral system is limited to a comparatively
small area, and the interambulacral system is wanting. The order of
Crinoids has diminished so much in modern geological times that we
must consult its fossil forms in order to understand fully the
peculiar adaptation of the Echinoderm plan in this group.
In the Ophiurans, the dorsal system is still large, and though it no
longer stretches out to form a stem, it folds over on the under side
of the animal so as to enclose entirely the ambulacral system, forming
a kind of shield for the arms. Here also the interambulacral system is
wanting.
In the Star-fishes the dorsal system encroaches less upon the
structure of the animal. The back and oral side here correspond
exactly in size, and though the flat leathery upper surface of the
animal, covered with spines, serves as a protection to the delicate
ambulacral suckers which find their way between the rows of small
plates along the under side of the arms, yet it does not enfold them
as in the Ophiurans. On the contrary, in the Star-fishes the
ambulacral rows are protected on either side by a row of the so-called
interambulacral plates, through which no suckers pass.
In the Sea-urchin, the dorsal system is contracted to a minimum,
forming a small area on the top of the animal, the rows of
interambulacral plates which are separated and lie on either side of
the ambulacra in the Star-fish being united in the Sea-urchin, and
both the ambulacral and the interambulacral systems bent upward,
meeting in the small dorsal area above, so as to form a spherical
outline. Here the ambulacral and interambulacral systems have taken a
great preponderance over the dorsal system, and the same is the case
with the Holothurians, in which the same structure is greatly
elongated, the dorsal system being thus pushed out as it were to the
end of a cylinder, while the ambulacral and interambulacral systems
run along its whole length. All Echinoderms without exception have
ambulacral tubes, even though in some there are no external ambulacral
suckers connected with them.
There is one organ peculiar to the class of Echinoderms, the general
structure of which may be described here, since it is common to them
all, with the exception of the Crinoids, the anatomy of which is,
however, so imperfectly understood, that we are hardly justified in
assuming that it does not exist even in that order. This organ is
known as the madreporic body; it is a small sieve or limestone filter
opening into a tube or canal; by means of this tube, which connects
with the ambulacral system, the water from without, first filtered
through the madreporic body and thus freed from any impurities, is
conveyed to the ambulacra. In the more detailed account of the
different orders we shall see what is the position of this singular
organ in each group, and how it is adapted in them all to their
special structure. The development of Echinoderms forms one of the
most wonderful chapters in the annals of Natural History. Marvellous
as is the embryonic history of the Acalephs, including all the
different aspects they assume in the cycle of their growth, it is
thrown into the shade by the transformations which Echinoderms undergo
before assuming their adult condition. This singular mode of
development, although it has features recalling the development of
Jelly-fishes from Hydroids, is nevertheless entirely distinct from it,
and is known only in the class of Echinoderms. As the whole story is
given at length in the chapter on the embryology of the Echinoderms,
we need only allude to it here in general terms. We owe the discovery
of this remarkable process to Johannes Müller, one of the greatest
anatomists of this century.
* * * * *
HOLOTHURIANS.
_Synapta_. (_Synapta tenuis_ AYRES.)
[Illustration: Fig. 124. Synapta, natural size.]
This is one of the most curious of the Holothurians, and easily
observed on account of its transparency, which allows us to see its
internal structure. It has a long cylindrical body (Fig. 124) along
the length of which run the five rows of ambulacra, which are in this
instance closed tubes without any projecting suckers or locomotive
organs of any kind attached to them, so that the name is retained only
on account of their correspondence in position, and not from any
similarity of function to the ambulacra in Star-fishes and
Sea-urchins. But though the ambulacra in Synapta are in fact mere
water-tubes like the vertical tubes in the Ctenophoræ, by means of
which the water, first filtered through the madreporic body,
circulates along the skin, they are as organs perfectly homologous
with the ambulacra in all other Echinoderms. The mouth has a circular
tube around the aperture, and a wreath of branching tentacles
encircling it. The habits of these animals are singular. They live in
very coarse mud, but they surround themselves with a thin envelope of
finer sand, which they form by selecting the small particles with
their tentacles, and making a ring around their anterior extremity.
This ring they then push down along the length of the body, and
continue this process, adding ring after ring, till they have entirely
encircled themselves with a sand tube. They move the rings down partly
by means of contractions of the body, but also by the aid of
innumerable appendages over the whole surface. To the naked eye these
appendages appear like little specks on the skin; but under the
microscope they are seen to be warts projecting from the surface, each
one containing a little anchor with the arms turned upward (Fig. 125).
Around the mouth these warts are larger, but do not contain any
anchors. It will be seen hereafter that these appendages are
homologous with certain organs in other Holothurians, the warts with
the anchors corresponding to the limestone pavement covering or
partially covering the surface of the Cuvieria, for instance, while
those without anchors correspond to the so-called false ambulacra in
Pentacta. By means of these appendages, though aided also by the
contractions of the body, the Synaptæ move through the mud and collect
around themselves the sand tube in which they are encased. Their food
is very coarse for animals so delicate in structure. When completely
empty of food they are white, perfectly transparent, and the spiral
tube forming the digestive cavity may be seen wound up and hanging
loosely in the centre for the whole length of the body. In such a
condition it is of a pale yellow color. But look at one that is gorged
with food. The whole length of the alimentary canal is then crowded
with sand, pebbles, and shells, distinctly seen through the
transparent skin, and giving a dark gray color to the whole body. They
swallow the sand for the sake of the nutritious substance it contains,
and having assimilated and digested this, they then eject the harder
materials. The motion of the body in consequence of its contractions
is much like that of leeches, and on this account these Synaptæ were
long supposed to be a transition type between the Radiates and worms.
The body grows to a great length, often half a yard and more, but
constantly drops large portions from its posterior part, by means of
its own contractions, or breaks itself up by the expulsion of the
intestines, which are very readily cast out. The tentacles are hollow,
consisting of a central rib with branches from either side. In the
Synaptæ, as in all the Holothurians, the madreporic body is placed
near the mouth, between two of the ambulacra, and opposite the fifth
or odd one. The tube, connecting with the central tube around the
mouth, by means of which it communicates with the ambulacral tubes, is
very short.
[Illustration: Fig. 125. Anchor of Synapta; _a_ anchor, _w_ plate
upon which anchor is attached; greatly magnified.]
_Caudina_. (_Caudina arenata_ STIMPS.)
[Illustration: Fig. 126. Caudina arenata; natural size.]
Several other Holothurians are frequently met with on our shores.
Among them is the _Caudina arenata_ (Fig. 126), a small Holothurian,
yellowish in color, and thick in texture, by no means so pretty as the
white transparent Synapta; the tentacles are short, resembling a crown
of cloves around the mouth. It lives in the sand, and may be found in
great numbers on the sandy beaches after a storm.
_Cuvieria_. (_Cuvieria squamata_ D. & K.)
[Illustration: Fig. 127. Cuvieria; natural size.]
The Holothurian of our coast, excelling all the rest in beauty, is the
Cuvieria. (Fig. 127.) As it lies on the sand, a solid red lump, with
neither grace of form nor beauty of color, even the vividness of its
tint growing dull and dead when it is removed from its native element,
certainly no one could suspect that it possessed any hidden charm; but
place it in a glass bowl with fresh sea-water; the dull red changes to
deep vivid crimson, the tentacles creep out (Fig. 127) softly, and
slowly, till the mouth is surrounded by a spreading wreath, comparable
for richness of tint, and for delicate tracery, to the most beautiful
sea-weeds. These tentacles, when fully expanded, are as long as the
body itself. A limestone pavement composed of numerous pieces covers
almost the whole surface of the animal; this apparatus corresponds, as
we have already mentioned, to the warts containing anchors in the
Synapta; but in the latter, the limestone particles are smaller,
whereas in the Cuvieria they are developed to a remarkable extent.
This animal is very sluggish, the ambulacral suckers, found only on
three of the tubes, being arranged in such a way as to form a sort of
sole on which they creep; the sole is tough and leathery in texture,
but free from the limestone pavement described above. The young (Figs.
128, 129) are very common, swimming freely about, and more readily
found than the adult; they are of a bright vermilion color, but the
tentacles hardly branch at that age, nor is the limestone pavement
formed, which gives such a peculiar aspect to the full-grown animal.
The young Cuvieria, somewhat older than that represented in Fig. 129,
are found in plenty under stones at low-water mark, just after they
have given up their nomadic habits, and when the limestone pavement
begins to be developed.
[Illustration: Fig. 128. Young Cuvieria, much enlarged; _l_ body,
_g_ tentacles.]
[Illustration: Fig. 129. Somewhat older Cuvieria; _l_ body, _g_
tentacle round mouth, _g'_ testaete of sole, _b_ madreporic
tentacle.]
_Pentacta_. (_Pentacta frondosa_ JÄG.)
The highest of our Holothurians in structure, is the Pentacta. (Fig.
130) It is very rare on our beaches, though occasionally found under
stones at low-water mark; farther north, in Maine, and at Grand Manan,
it is very common, covering all the rocks near low-water mark. It is a
chocolate brown in color, and measures, when fully expanded, some
fifteen to eighteen inches in length. Unlike the Cuvieria, the
ambulacral suckers are evenly distributed and almost equally developed
on all the tubes; between the five rows of ambulacral suckers are
scattered irregularly certain appendages resembling suckers, but found
on examination not to be true locomotive suckers, and called on that
account false ambulacra. These are the organs corresponding to the
warts around the mouth of the Synapta. Although the ambulacral suckers
are, as we have said, equally developed on all the tubes, yet the
Pentacta does not use them indiscriminately as locomotive organs. In
Pentacta, as well as in all Holothurians, whether provided with
ambulacral suckers, or, like the Synapta and Caudina, deprived of
them, the odd ambulacrum, viz. the one placed opposite the madreporic
body, is always used to creep upon, and forms the under surface of the
animal.
[Illustration: Fig. 130. Pentacta frondosa; expanded about one
third the natural size.]
The correspondence between the different phases of growth in the young
Pentacta, and the adult forms of the orders described above, the
Synapta, Caudina, Cuvieria, and Pentacta itself, is a striking
instance of the way in which embryonic forms illustrate the relative
standing of adult animals. In the earlier stages of its development,
the ambulacral tubes alone are developed in the Pentacta; in this
condition it recalls the lower orders of Holothurians, as the Synapta
and Caudina; then a sole is formed by the greater development of three
of the ambulacra, and in this state it reminds us of the next in
order, the Cuvieria, while it is only in assuming its adult form that
the Pentacta develops its other ambulacra, with their many suckers.
The Pentacta resembles the Trepang, so highly valued by the Chinese as
an article of food, and forms a not unsavory dish, having somewhat the
flavor of lobster.
* * * * *
ECHINOIDS.
_Sea-urchin_. (_Toxopneustes drobachiensis_ AG.)
[Illustration: Fig. 131. Toxopneustes from above, with all the
appendages expanded; natural size.]
Sea-urchins (Fig. 131) are found in rocky pools, hidden away usually
in cracks and holes. They like to shelter themselves in secluded
nooks, and, not satisfied even with the privacy of such a retreat,
they cover themselves with sea-weed, drawing it down with their
tentacles, and packing it snugly above them, as if to avoid
observation. This habit makes them difficult to find, and it is only
by parting the sea-weed, and prying into the most retired corners in
such a pool, that one detects them. Their motions are slow, and they
are less active than either the Star-fish or the Ophiuran, to both of
which they are so closely allied.
Let us look at one first, as seen from above, with all its various
organs fully extended. (Fig. 131.) The surface of the animal is
divided by ten zones, like ribs on a melon, only that these zones
differ in size, five broad zones alternating with five narrower ones.
The broad zones, representing the interambulacral system, are composed
of large plates, supporting a number of hard projecting spines, while
the narrow zones, forming the ambulacral system, are pierced with
small holes, arranged in regular rows, (Fig. 132,) through which
extend the tentacles terminating with little cups or suckers. These
zones converge towards the summit of the animal, meeting in the small
area which here represents the dorsal system; this area is filled by
ten plates, five larger ones at the extremity of the interambulacral
zones, and five smaller ones at the extremity of the ambulacral zones.
(Fig. 132.) In the five larger plates are the ovarian openings, so
called because each one is pierced by a small hole through which the
eggs are passed out, while in the five smaller plates are the
eye-specks. The ovaries themselves consist of long pouches or sacs,
carried along the inner side of each ambulacrum; one of these ovarian
plates is larger than the others, and forms the madreporic body, being
pierced with many minute holes; here, as in the Star-fish, it is
placed between two of the ambulacral rows, and opposite the fifth or
odd one. Looked at from the under or the oral side, as seen in Fig.
134, the animal presents the mouth, a circular aperture furnished with
five teeth in its centre; these five teeth opening into a complicated
intestine to be presently described. From the mouth, the ten zones
diverge, curving upward to meet in the dorsal area on the summit of
the body. (Fig. 133.)
[Illustration: Fig. 132. Portion of shell of Fig. 131, with spines
rubbed off. (_Agassiz_.)]
[Illustration: Fig. 133. Sea-urchin shell with all the spines
removed. (_Agassiz_.)]
Let us now examine the appearance and functions of the various
appendages on the surface. The tentacles have a variety of functions
to perform; they are the locomotive appendages, and for this reason,
as we have seen, the zones along which they are placed are called the
ambulacra, while the intervening spaces, or the broad zones, are
called the interambulacra. It should not be supposed, however, that
the locomotive appendages are the only ones to be found on the
ambulacra, for spines occur on the narrow as well as on the broad
zones, though the larger and more prominent ones are always placed on
the latter. The tentacles are also subservient to circulation, for the
water which is taken in at the madreporic body passes into all the
tentacles, sometimes called on that account water-tubes. Beside these
offices the tentacles are constantly busy catching any small prey, and
conveying it to the mouth, or securing the bits of sea-weed with
which, as has been said, these animals conceal themselves from
observation. It is curious to see their fine transparent feelers,
fastening themselves by means of the terminal suckers on such a
floating piece of sea-weed, drawing it gently down and packing it
delicately over the surface of the body. As locomotive appendages, the
tentacles are chiefly serviceable on the lower or oral side of the
animal, which always moves with the mouth downward. About this portion
of the body the tentacles are numerous (Fig. 134) and large, and when
the animal advances it stretches them in a given direction, fastens
them by means of the suckers on some surface, be it of rock, or shell,
or the side of the glass jar in which they are kept, and being thus
anchored it drags itself forward. The tentacles are of a violet hue,
though when stretched to their greatest length they lose their color,
and become almost white and transparent; but in their ordinary
condition the color is quite decided, and the rows along which they
occur make as many violet lines upon the surface of the body.
[Illustration: Fig. 134. Sea-urchin seen from the mouth side.
(_Agassiz_.)]
[Illustration: Fig. 135. Magnified spine.]