I will take, then, for the further illustration of my subject, the Radiates, and especially the class of Echinoderms, Star-Fishes, Sea-Urchins, and the like, both in the fossil and the living types; and though some special description of these animals is absolutely essential, I will beg my readers to remember that the general idea, and not its special manifestations, is the thing I am aiming at, and that, if we analyze the special parts characteristic of these different groups, it is only that we may resolve them back again into the structural plan that includes them all.
I have already in a previous article named the different Orders of this Class in their relative rank, and have compared the standing of the living ones, according to the greater or less complication of their structure, with the succession of the fossil ones. Of the five Orders, Beches-de-Mer, Sea-Urchins, Star-Fishes, Ophiurans, and Crinoids,—or, to name them all according to their scientific nomenclature, Holothurians, Echinoids, Asteroids, Ophiurans, and Crinoids,—the last-named are lowest in structure and earliest in time. Cuvier was the first naturalist who detected the true nature of the Crinoids, and placed them where they belong in the classification of the Animal Kingdom. They had been observed before, and long and laborious investigations had been undertaken upon them, but they were especially baffling to the student, because they were known only in the fossil condition from incomplete specimens; and though they still have their representatives among the type of Echinoderms as it exists at present, yet, partly owing to the rarity of the living specimens and partly to the imperfect condition of the fossil ones, the relation between them was not recognized. The errors about them certainly did not arise from any want of interest in the subject among naturalists, for no less than three hundred and eighty different authors have published their investigations upon the Crinoids, and the books that have been printed about these animals, many of which were written long before their animal nature was suspected, would furnish a library in themselves. The ancients knew little about them. The only one to be found in the European seas resembles the Star-Fish closely, and they called it Asterias; but even Aristotle was ignorant of its true structural relations, and alludes only to its motion and general appearance. Some account of the gradual steps by which naturalists have deciphered the true nature of these lowest Echinoderms and their history in past times may not be without interest, and is very instructive as showing bow such problems may be solved.
In the sixteenth century some stones were found bearing the impression of a star on their surface. They received the name of Trochites, and gave rise to much discussion. Naturalists puzzled their brains about them, called them star-shaped crystals, aquatic plants, corals; and to these last Linnaeus himself, the great authority of the time on all such questions, referred them. Beside these stony stars, which were found in great quantities when attention was once called to them, impressions of a peculiar kind had been observed in the rocks, resembling flowers on long stems, and called "stone lilies" naturally enough, for their long, graceful stems, terminating either in a branching crown or a closer cup, recall the lily tribe among flowers. The long stems of these seeming lilies are divided transversely at regular intervals;—the stem is easily broken at any of these natural divisions, and on each such fragment is stamped a star-like impression resembling those found upon the loose stones or Trochites.
About a century ago, Guettard the naturalist described a curious specimen from Porto Rico, so similar to these fossil lilies of the rocks that he believed they must have some relation to each other. He did not detect its animal nature, but from its long stem and branching crown he called it a marine palm. Thus far neither the true nature of the living specimen, nor of the Trochites, nor of the fossil lilies was understood, but it was nevertheless an important step to have found that there was a relation between them. A century passed away, and Guettard's specimen, preserved at the Jardin des Plantes, waited with Sphinx-like patience for the man who should solve its riddle.
Cuvier, who held the key to so many of the secrets of Nature, detected at last its true structure; he pronounced it to be a Star-Fish with a stem, and at once the three series of facts respecting the Trochites, the fossil lilies, and Guettard's marine palm assumed their true relation to each other. The Troehites were recognized as simply the broken portions of the stem of some of these old fossil Crinoids, and the Crinoids themselves were seen to be the ancient representatives of the present Comatulae and Star-Fishes with stems. So is it often with the study of Nature; many scattered links are collected before the man comes who sees the connection between them and speaks the word that reconstructs the broken chain.
I will begin my comparison of all Echinoderms with an analysis of the Star-Fishes and Sea-Urchins, because I think I can best show the identity of parts between them, notwithstanding the difference in their external form; the Sea-Urchins having always a spherical body, while the Star-Fishes are always star-shaped, though in some the star is only hinted at, sketched out, as it were, in a simply pentagonal outline, while in others the indentations between the rays are very deep, and the rays themselves so intricate in their ramifications as to be broken up into a complete net-work of branches. But under all this variety of outline, our problem remains always the same: to build with the same number of pieces a star and a sphere, having the liberty, however, of cutting the pieces differently and changing their relative proportions. Let us take first the Sea-Urchin and examine in detail all parts of its external structure. I shall say nothing of the internal structure of any of these animals, because it does not affect the comparison of their different forms and the external arrangement of parts, which is the subject of the present article.
On the lower side is the mouth, and we may call that side and all the parts that radiate from it the oral region. On the upper side is a small area to which the parts converge, and which, from its position just opposite the so-called mouth or oral opening, we may call the ab-oral region. I prefer these more general terms, because, if we speak of the mouth, we are at once reminded of the mouth in the higher animals, and in this sense the word, as applied to the aperture through which the Sea-Urchins receive their food, is a misnomer. Very naturally the habit has become prevalent of naming the different parts of animals from their function, and not from their structure; and in all animals the aperture through which food enters the body is called the mouth, though there is not the least structural relation between the organs so designated, except within the limits of each different branch or division. To speak of these opposite regions in the Sea-Urchin as the upper and lower sides would equally mislead us, since, as we have seen, there is, properly speaking, no above and below, no right and left sides, no front and hind extremities in these animals, all parts being evenly distributed around a vertical axis. I will, therefore, although it has been my wish to avoid technicalities as much as possible in these papers, make use of the unfamiliar terms oral and ab-oral regions, to indicate the mouth with the parts diverging from it and the opposite area towards which all these parts converge. [Footnote: When reference is made to the whole structure, including the internal organs as well as the solid parts of the surface, the terms actinal and ab-actinal are preferable to oral and ab-oral.]
[Illustration: Sea-Urchin seen from the oral side, showing the zones with the spines and suckers; for the ab-oral side, on the summit of which the zones unite, see February Number, p. 216.]
The whole surface of the animal is divided by zones,—ten in number, five broader ones alternating with five narrower ones. The five broad zones are composed of large plates on which are the most prominent spines, attached to tubercles that remain on the surface even when the spines drop off after death, and mark the places where the spines have been. The five small zones are perforated with regular rows of holes, and through these perforations pass the suckers or water-tubes which are their locomotive appendages. For this reason these narrower zones are called the ambulacra, while the broader zones intervening between them and supporting the spines are called the interambulacra. Motion, however, is not the only function of these suckers; they are subservient also to respiration and circulation, taking in water, which is conveyed through them into various parts of the body.
[Illustration: Portion of Sea-Urchin representing one narrow zone with a part of the broad zones on either side and the ab-oral area on the summit.]