Chapter IV: Part 4

The various kinds present very different aspects; wherever the larger Aureliæ and Zygodactylæ float to the surface, they bring with them a dim spreading halo of light, the smaller Ctenophoræ become little shining spheres, while a thousand lesser creatures add their tiny lamps to the illumination of the ocean; for this so-called phosphorescence of the sea is by no means due to the Jelly-fishes alone, but is also produced by many other animals, differing in the color as well as the intensity of their light, and it is a curious fact that they seem to take possession of the field by turns. You may row over the same course, which a few nights since glowed with a greenish golden light wherever the surface of the water was disturbed, and though equally brilliant, the phosphorescence has now a pure white light. On such an evening, be quite sure that when you empty your buckets on your return and examine their contents you will find that the larger part of your treasures are small crustacea (little shrimps). Of course there will be other phosphorescent creatures, Jelly-fishes, &c., among them, but the predominant color is given by these little crustacea. On another evening the light will have a bluish tint, and then the phosphorescence is principally due to the Dysmorphosa (Fig. 105).

Notwithstanding the beauty of a moonlight row, if you would see the 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.

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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.

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HOLOTHURIANS.

_Synapta_. (_Synapta tenuis_ AYRES.)

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.

_Caudina_. (_Caudina arenata_ STIMPS.)

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.)

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.

_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.

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.

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ECHINOIDS.

_Sea-urchin_. (_Toxopneustes drobachiensis_ AG.)

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.)

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.

Almost the sole function of the spines seems to be that of protecting the animal, and enabling it to resist the attacks of its enemies, the force of the waves, or any sudden violent contact with the rocks. The spines, when magnified, are seen to be finely ribbed for nearly the whole length (Fig. 135), the bare basal knob serving as the point of attachment for the powerful muscles, which move these spines on a regular ball-and-socket joint, the ball surmounting the tubercles (seen in Fig. 132), which fit exactly in a socket at the base of the spine. In a transverse section of a spine (Fig. 136), we see that the ribs visible on the outside are delicate columns placed closely side by side, and connected by transverse rods forming an exceedingly delicate pattern. Beside the tentacles and the spines, they have other external appendages, of which the function long remained a mystery, and is yet but partially explained; these are the so-called pedicellariæ; they consist of a stem (_s_, Fig. 137), which becomes swollen (_p_, Fig. 137) into a thimble-shaped knob at the end (_t_, Fig. 137); this knob may seem solid and compact at first sight, but it is split into three wedges, which can be opened and shut at will. When open, these pedicellariæ may best be compared to a three-pronged fork, except that the prongs are arranged concentrically instead of on one plane, and, when closed, they fit into one another as neatly as the pieces of a puzzle.

If we watch the Sea-urchin after he has been feeding, we shall learn, at least, one of the offices which this singular organ performs in the general economy of the animal. That part of his food which he ejects passes out at an opening on the summit of the body, in the small area where all the zones converge. The rejected particle is received on one of these little forks, which closes upon it like a forceps, and it is passed on from one to the other, down the side of the body, till it is dropped off into the water. Nothing is more curious and entertaining than to watch the neatness and accuracy with which this process is performed. One may see the rejected bits of food passing rapidly along the lines upon which these pedicellariæ occur in greatest number, as if they were so many little roads for the conveying away of the refuse matters; nor do the forks cease from their labor till the surface of the animal is completely clean, and free from any foreign substance. Were it not for this apparatus the food thus rejected would be entangled among the tentacles and spines, and be stranded there till the motion of the water washed it away. These curious little organs may have some other office than this very laudable and useful one of scavenger, and this seems the more probable because they occur over the whole surface of the body, while they seem to pass the excrements only along certain given lines. They are especially numerous about the mouth, where they certainly cannot have this function; we shall see also that they bear an important part in the structure of the Star-fish, where there are no such avenues on the upper surface, for the passage of the refuse food, as occur on the Sea-urchin.

On opening a Sea-urchin, we find that the teeth (Fig. 138), which seem at first sight only like five little conical wedges around the mouth (Fig. 134), are connected with a complicated intestine, which extends spirally from the lower to the upper floor of the body, festooning itself from one ambulacral zone to the next, till it reaches the summit, where it opens. This intestine leads into the centre of the teeth, the jaws themselves, which sustain the teeth, being made up of a number of pieces, and moved by a complicated system of muscular bands. When the intestine is distended with food, it fills the greater part of the inner cavity; the remaining space is occupied in the breeding season by the genital organs. In a section of the Sea-urchin, one may also trace the tube by which the supply of water, first filtered through the madreporic body, is conveyed to the ambulacra; it extends from the summit of the body to the circular tube surrounding the mouth.

_Echinarachnius_. (_Echinarachnius parma_ GRAY.)

Beside the Toxopneustes (Fig. 131) described above, we have another Sea-urchin very common along our shores. Among children who live near sandy beaches, they are well known as "sand-cakes" (Fig. 139), and indeed they are so flat and round, that, when dried and deprived of their bristles, they look not unlike a cake with a star-shaped figure on its surface. (Fig. 139.) When first taken from the water they are of a dark reddish brown color, and covered with small silky bristles. The disk is so flat, being but very slightly convex on the upper side, that one would certainly not associate it at first sight with the common spherical Sea-urchin or Sea-egg, as the Toxopneustes is sometimes called. But upon closer examination the delicate ambulacral tubes or suckers may be seen projecting from along the line of the ambulacra, as in the spherical Sea-urchin; and though these ambulacra become expanded near the summit into gill-like appendages, forming a sort of rosette in the centre of the disk, they are, nevertheless, the same organs, only somewhat more complicated. When such a disk is dried in the sun, and the bristles entirely removed, the lines of suture of the plates composing it, and corresponding exactly to those of the spherical Sea-urchin, may very readily be seen. (_a_ and _i_, Fig. 139.)

This flat Sea-urchin or Echinarachnius, as it is called, belongs to a group of Sea-urchins known as Clypeastroids (shield-like Sea-urchins). In a section (Fig. 140) exposing the internal structure, one cannot but be reminded by its general aspect of an Aurelia. Could one solidify an Aurelia it would present much the same appearance; another evidence that all the Radiates are built on one plan, their differences being only so many modes of expressing the same structural idea. The teeth or jaws in this flat Sea-urchin are not so complicated as in the Toxopneustes, being simply flat pieces, arranged around the mouth (_o_, Fig. 140), without the apparatus of muscular bands by means of which the teeth are moved in the other genus. It is a curious fact, considered in relation to the general radiate structure of these animals, that the teeth, instead of moving up and down like the jaws in Vertebrates, or from right to left like those of Articulates, move concentrically, all converging towards the centre.

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STAR-FISHES.

_Star-fish_. (_Astracanthion berylinus_ AG.)

Although there is the closest homology of parts between the Star-fish and the Sea-urchin, the arrangement of these parts, and the external appearance of the animals, as a whole, are entirely different. The Star-fish has zones corresponding exactly to those of the Sea-urchin, but instead of being drawn together, and united at the summit of the animal, so as to form a spherical outline, they are spread out on one level in the shape of a star. This change in the general arrangement brings the eye-specks to the extremities of the arms, and places the ovarian openings in the angles between the arms. The madreporic body is situated on the upper surface of the disk (Fig. 142), at the angle between two of the arms, and consequently between two of the ambulacra, and opposite the odd one. The tube into which it opens, runs vertically from the upper floor of the disk to the lower, where it connects with the circular tube around the mouth, and thus communicates with all the ambulacral rows. The ambulacral zones which, in the Star-fish, have the shape of a furrow, run along the lower side of each ray (Fig. 141); the interambulacral zones are divided, their plates being arranged in rows along either side of the ambulacral furrows. The ambulacral furrow, like the ambulacral zone in the Sea-urchin, is pierced with numerous holes, alternating with each other in a kind of zigzag arrangement, one hole a little in advance, the next a little farther back, and so on, and through these holes pass the tentacles, terminating in suckers, as in the Sea-urchins, and serving as in them for locomotive organs. The most prominent and strongest spines are arranged upon the large interambulacral plates on both sides of the ambulacral furrows; but the upper surface of the animal is also completely studded with smaller spines, scattered at various distances, apparently without any regular arrangement. (Fig. 142.)

The position of the pedicellariæ is quite different from that which they occupy in the Sea-urchin, where they are scattered singly between the spines and tentacles, though more regularly and closely grouped along the lines upon which the refuse food is moved off. In the Star-fish, on the contrary, these singular organs seem to be grouped for some special purpose around the spines, on the upper surface of the body. Every such spine swells near its point of attachment, thus forming a spreading base (Fig. 143), around which the pedicellariæ are arranged in a close wreath, in the centre of which the summit of the spine projects; they differ also from those of the Sea-urchin in having two prongs instead of three. Other pedicellariæ are scattered independently over the surface of the animal, but they are smaller than those forming the clusters and connected with the spines. The function of these organs in the Star-fish remains unexplained; the opening on the upper surface, through which the refuse food is thrown out, is in such a position that they evidently do not serve here the same purpose which renders them so useful to the Sea-urchin. Occasionally they may be seen to catch small prey with these forks, little Crustacea, for instance; but this is probably not their only office. The Star-fish has a fourth set of external appendages in the shape of little water-tubes. (Seen in Fig. 143.) The upper surface of the back consists of a strong limestone network (Fig. 144), and certain openings in this network are covered with a thin membrane through which these water-tubes project. It is supposed that water may be introduced into the body through these tubes; but while there can be no doubt that they are constantly filled with water, and are therefore directly connected with the circulation through the madreporic body (Fig. 145), no external opening has as yet been detected in them. The fact, however, that when these animals are taken out of their native element, the water pours out of them all over the surface of the back, so that they at once collapse and lose entirely their fulness of outline, seems to show that water does issue from those tubes. The ends of the arms are always slightly turned up, and at the summit of each is a red eye-speck. The tentacles about the eye become very delicate and are destitute of suckers.

These animals have singular mode of eating; they place themselves over whatever they mean to feed upon, as a cockle-shell for instance, the back gradually rising as they arch themselves above it; they then turn the digestive sac or stomach inside out, so as to enclose their prey completely, and proceed leisurely to suck out the animal from its shell. Cutting open any one of the arms we may see the yellow folds of the stomach pouches which extend into each ray; within the arms, extending along either side of the upper surface, are also seen the ovaries, like clusters of small yellow berries. Immediately below these, along the centre of the lower floor of each ray, runs the ridge formed by the ambulacral furrow, and upon either side of this ridge are placed the vesicles, by means of which the tentacles may be filled and emptied at the will of the animal; the rest of the cavity of the ray is filled by the liver. The mouth, which is surrounded by a circular tube, is not furnished with teeth, as in the Sea-urchin; but the end of each ambulacral ridge is hard, thus serving the purpose of teeth.

_Cribrella_. (_Cribrella oculata_ FORBES.)

Our coast, as we have said, is not rich in the variety of Star-fishes. We have two large species, one of a dark-brown color (Fig. 132), the _Astracanthion berylinus_, and the other, the _A. pallidus_, of a pinkish tint; then there is the small Cribrella, inferior in structural rank to the two above mentioned. (Fig. 146.) This pretty little Star-fish presents the greatest variety of colors; some are dyed in Tyrian purple, others have a paler shade of the same hue, some are vermilion, others a bright orange or yellow. A glass dish filled with Cribrellæ might vie with a tulip-bed in gayety and vividness of tints.

The disk of the Cribrella is smooth, instead of being covered, like the larger Star-fishes, with a variety of prominent appendages. The spines are exceedingly short, crowded like little warts over the surface. It is an interesting fact, illustrating again the correspondence between the adult forms of the lower orders and the phases of growth in the higher ones, that these spines have an embryonic character. One would naturally expect to find that these small spines of the adult Cribrella would differ from those of the other full-grown Star-fishes chiefly in size, that they would be a somewhat modified pattern of the same thing on a smaller scale; but when examined under the microscope, they resemble the spines of the higher orders in their embryonic condition; it is not, in fact, a difference in size merely, but a difference in degree of development. The Cribrella moves usually with two of the arms turned backward, and the three others advanced together, the two posterior ones being sometimes brought so close to each other as to touch for their whole length.

_Hippasteria_. (_Hippasteria phrygiana_ AG.)

Beside these Star-fishes we have the pentagonal Hippasteria (_Hippasteria phrygiana_ AG.), like a red star with rounded points, found chiefly in deep water, though it is occasionally thrown up on the beaches. It has but two rows of large tentacles, terminating in a powerful sucking disk. The pedicellariæ on this Star-fish resemble large two-pronged clasps, arranged principally along the lower side. The pentagonal Star-fishes of our coast are in striking contrast to the long-armed species we have just described; they are edged with rows of large smooth plates, and do not possess the many prominent spines so characteristic of the ordinary Star-fishes.

_Ctenodiscus_. (_Ctenodiscus crispatus_ D. & K.)

The Ctenodiscus (_Ctenodiscus crispatus_ D. & K., Fig. 147), an inhabitant of more northern waters, but seeming also to be at home here occasionally, is another pentagonal Star-fish. It lives in deep water, and frequents muddy bottoms. The peculiar structure of their ambulacra has probably some reference to this mode of living, for they are entirely wanting in the sucking disks so characteristic of the other members of this class, and their tentacles are pointed, as if to enable them to work their way through the mud in which they make their home. The pointed tentacles of this genus are characteristic of a large group of Star-fishes, and it is an important fact, as showing their lower standing, that this feature, as well as the pentagonal outline, obtains in the earlier stages of growth of our more common Star-fishes, while in their adult condition they assume the deeply indented star-shaped outline, and have suckers at the extremities of the tentacles.

_Solaster_. (_Solaster endeca_ FORBES.)

We find also among Star-fishes the same tendency to multiplication of parts so common among the Polyps and Acalephs. Our Solaster (_Solaster endeca_ Forbes), for instance, has no less than twelve arms; it inhabits more northern latitudes, though sometimes found in our Bay; on the coast of Maine it is quite common, and occurs in company with another many-rayed species, the _Crossaster papposa_ M. & T. The color of both of these Star-fishes is exceedingly varied; we find in the Solaster as many different hues as in the Cribrella, which it resembles in the structure of its spines, while in the Crossaster bands of different tints of red and purple are arranged concentrically, and the whole surface of the back is spotted with brilliantly-tinged tiny wreaths of water-tubes, crowded round the base of the different spines, which are somewhat similar to those of the Astracanthion.

OPHIURANS.

_Ophiopholis_. (_Ophiopholis bellis_ LYM.)

There are but two species of the ordinary forms of Ophiurans in Massachusetts Bay; the white Amphiura (_Amphiura squamata_ Sars), with long slender arms, and the spotted Ophiopholis (Fig. 148), with shorter and stouter arms, and in which the disk is less compact than in the Amphiura, and not so perfectly circular. All Ophiurans are difficult to find, from their exceeding shyness; they hide themselves in the darkest crevices, and though no eye-specks have yet been detected in them, they must have some quick perception of coming danger, for at the gentlest approach they instantly draw away and shelter themselves in their snug retreats.

They differ from the Star-fishes in having the disk entirely distinct from the arms; that is, the arms, instead of merging gradually into the disk, start at once from its margin. They have no interambulacral spaces or plates; but the whole upper surface is formed of large hard plates, which extend from the back over the sides of the arms to their lower surface, where they form a straight ridge along the centre. (Fig. 149.) The sides of these plates are pierced with holes, through which the tentacles pass; these have not, like those of the Star-fishes and Sea-urchins, a sucker at the extremity, but are covered with little warts or tubercles (Fig. 150); they are their locomotive appendages, and their way of moving is curious; they first extend one of the arms in the direction in which they mean to move, then bring forward two others to meet them, three arms being thus usually in advance, and then they drag the rest of the body on. They move with much more rapidity, and seem more active, than the Star-fishes; probably owing to the greater independence of the arms from the disk. The spines project along the margin of the arms, and not over the whole surface, the back of the arms being perfectly free from any appendages, and presenting only the surface of the plates. The madreporic body is formed by a plate on the lower side of the disk, in a position corresponding to that which it occupies in the young Star-fish; this plate is one of the large circular shields occupying the interambulacral spaces around the mouth. (Fig. 149.) On each side of the arms, where they join the disk, are slits opening into the ovarian pouches. They have no teeth; but the hard ridge at the oral end of the ambulacra, extending toward the mouth in Star-fish, is still more distinct and sharper in the Ophiurans, approaching more nearly the character of teeth.

_Astrophyton_. (_Astrophyton Agassizii_ STIMP.)

A singular species of Ophiuran, known among fishermen as the "Basket-fish," (Fig. 151,) is to be found in Massachusetts Bay. Its arms are very long in comparison to the size of the disk, and divide into a vast number of branches. In moving, the animal lifts itself on the extreme end of these branches, standing as it were on tiptoe (Fig. 151), so that the ramifications of the arms form a kind of trellis-work all around it, reaching to the ground, while the disk forms a roof. In this living house with latticed walls small fishes and other animals are occasionally seen to take shelter; but woe to the little shrimp or fish who seeks a refuge there, if he be of such a size as to offer his host a tempting mouthful; he will fare as did the fly who accepted the invitation of the spider. These animals are exceedingly voracious, and sometimes, in their greediness for food, entangle themselves in fishing lines or nets. When disturbed, they coil their arms closely around the mouth, assuming at such times a kind of basket-shape, from which they derive their name.

This Basket-fish is honorably connected with our early colonial history, being thought worthy, by no less a personage than John Winthrop, Governor of Connecticut, who, as he says, "had never seen the like," to be sent with "other natural curiosities of these parts" to the Royal Society of London, in 1670. He accompanies the specimen with a minute description, omitting "other particulars, that we may reflect a little upon this elaborate piece of nature." His account is as graphic as it is accurate, and we can hardly give a better idea of the animal than by extracting some portions of it. "This Fish," he says, "spreads itself from a Pentagonal Root, which incompasseth the Mouth (being in the middle), into 5 main Limbs or branches, each of which, just at issuing out from the Body, subdivides itself into two, and each of these 10 branches do again divide into two parts, making 20 lesser branches; each of which again divide into two smaller branches, making in all 40. These again into 80, and these into 160; and these into 320; these into 640; into 1280; into 2560; into 5120; into 10,240; into 20,480; into 40,960; into 81,920; beyond which the further expanding of the Fish could not be certainly trac'd";--a statement which we readily believe, wondering only at the patience which followed this labyrinth so far.

In a later letter, after having had an interview with the fisherman who caught the specimen, and, as he says, "asked all the questions I could think needful concerning it," the Governor proceeds to tell us that it was caught "not far from the Shoals of Nantucket (which is an Island upon the Coast of New England)," and that when "first pull'd out of the water it was like a basket, and had gathered itself round like a Wicker-basket, having taken fast hold upon that bait on the hook which he" (the fisherman) "had sunk down to the bottom to catch other Fish, and having held that within the surrounding brachia would not let it go, though drawn up into the Vessel; until, by lying a while on the Deck, it felt the want of its natural Element; and then voluntarily it extended itself into the flat round form, in which it appear'd when present'd to your view." The Governor goes on to reflect in a philosophical vein upon the purpose involved in all this complicated machinery. "The only use," he says, "that could be discerned of all that curious composure wherewith nature had adorned it seems to be to make it as a purse-net to catch some other fish, or any other thing fit for its food, and as a basket of store to keep some of it for future supply, or as a receptacle to preserve and defend the young ones of the same kind from fish of prey; if not to feed on them also (which appears probable the one or the other), for that sometimes there were found pieces of Mackerel within that concave. And he, the Fisherman, told me that once he caught one, which had within the hollow of its embracements a very small fish of the same kind, together with some piece or pieces of another fish, which was judged to be of a Mackerel. And that small one ('tis like) was kept either for its preservation or for food to the greater; but, being alive, it seems most likely it was there lodged for safety, except it were accidentally drawn within the net, together with that piece of fish upon which it might be then feeding." The account concludes by saying, "This Fisherman could not tell me of any name it hath, and 'tis in all likelihood yet nameless, being not commonly known as other Fish are. But until a fitter _English_ name be found for it, why may it not be called (in regard of what hath been before mentioned of it) a _Basket-Fish_, or a _Net-Fish_, or a _Purs-net-Fish_?" And so it remains to this day as the Governor of Connecticut first christened it, the Basket-fish.

* * * * *

CRINOIDS.

The Crinoids are very scantily represented in the present creation. They had their day in the earlier geological epochs, when for some time they remained the sole representatives of their class, and were then so numerous that the class of Echinoderms, with only one order, seemed as full and various as it now does with five. The different forms they assumed in the successive geological periods are particularly instructive; these older Crinoids combined characters which foreshadowed the advent of the Ophiurans, the true Star-fishes, and the Sea-urchins; and so prominently were their prophetic characters developed, that many of them are readily mistaken for Star-fishes or Sea-urchins.

In later times the group of Crinoids has been gradually dwindling in number and variety. Its present representatives are the Pentacrini of Porto Rico and the coast of Portugal, the lovely little Rhizocrinus of the Atlantic, dredged first by the younger Sars on the coast of Norway, attached throughout life to a stem, and the Comatula, which has a stem only in the early stages of its growth, but is free when adult. The Pentacrinus bears the closer relation to the more ancient Crinoids (Fig. 152), which were always supported on a stem, while it is only in more recent periods that we find the free Crinoids, corresponding to the Comatula.

_Comatula_. (_Alecto meridionalis_ AG.)

One large species of Comatula (_Alecto Eschrichtii_ M. & T.) is known on our coast, off the shores of Greenland, where it has been dredged at a depth of about one hundred and fifty fathoms, and young specimens of the same species have been found as far south as Eastport, Maine. The species selected for representation here, however, (Fig. 153,) is one quite abundant along the shores of South Carolina. It is introduced instead of the northern one, because the latter is so rare that it is not likely to fall into the hands of our readers. The annexed drawing (Fig. 154, magnified from Fig. 153) represents a group of the young of the Charleston Comatula, still attached to the parent body by their stems, and in various stages of development. At first sight, the Comatula, or, as it is sometimes called, the feather-star, resembles an Ophiuran; but on a closer examination we find that the arms are made up of short joints; and along the sides of the arms, attached to each joint, are appendages resembling somewhat the beards of a feather, and giving to each ray the appearance of a plume; hence the name of feather-star. On one side the arms are covered with a tough skin, through which project the ambulacræ, and on the same side of the disk are situated the mouth and the anus; the latter projects in a trumpet-shaped proboscis. On the opposite side of the disk the Comatula is covered with plates, arranged regularly around a central plate, which is itself covered with long cirri.