CHAPTER IX.
ECHINODERMATA.
"Ultra magis pisces et Echinos æquora celent."—Hor. Ep.
In their "Natural History of the Echinodermata," Messrs. Hupé and Dujardin divide this vast natural group into five orders or families, namely: 1, Asteroïdæ, which includes the true star-fishes; 2, Crinoïdæ, stone lilies, calcareous, stem composed of movable pieces; 3, Ophiuræ, having the disk much depressed, the rays simple, and furnished with short stems; 4, Echinidæ, comprehending the animals known as sea-eggs, or sea-urchins, distinguished by their rounded form and absence of arms; 5, Holothuroïdæ, with soft lengthened cylindrical body, covered with scattered suckers.
The Echinodermata, from the Greek words ἐχῖνοϛ, rough, and δέρμα, skin; indicating an animal bristling with spines like the hedgehog's. They are animals sometimes free, sometimes attached by a stem, flexible or otherwise, and radiating, that is, presenting an appearance more or less regular in all its parts, after the manner of a circle or star, its form being globular, egg-shaped, cylindrical, or like a pentagonal plate; or, lastly, like a star, with more or less elongated branches, which secrete either in all their tissues or only in the integument very numerous symmetrical calcareous plates of solid matter, sometimes forming an internal skeleton or regular shell covered with a more or less consistent skin, often pierced with holes, from which the feet or tentacula issue; they are frequently furnished with appendices of various kinds, such as prickles, scales, &c.
The organization of the Echinodermata is the most perfect of all the zoophytes, serving as a transition between them and animals of more complicated frame. They have a digestive and vascular system, and a muscular system is almost always present; in short, they have internal or external respiratory organs, and a rudimentary nervous system has been detected in many of the species. The nutritive system is very simple, presenting in most of the family a single orifice in the centre of the lower surface of the body, destitute of teeth, performing the functions both of mouth and anus. De Blainville says that "the liver is apparent and rather considerable in the star-fishes, forming bunches occupying the whole circumference of the stomach, and extending to the cavities of the appendages where these exist." The mouth and gullet is admirably adapted for securing the testaceous mollusks and other substances on which they feed.
Reproduction in the Echinodermata appears to be monœcious. Ovaries are, as far as is known, the only organs of generation. They vary in number in different species. The sexes are usually separate: the young are produced by eggs, the embryo of which undergo important metamorphoses. Immediately after birth, the young asteriæ have a depressed and rounded body, with four club-shaped appendages or arms at their anterior extremity. When they are a little more developed, papillæ may be observed on the upper surface, in fine radiating rows: after twelve days the fine rays begin to increase, and after eight days more two rows of feet, or tentacula, are developed under each ray, which assist in the locomotion of the animal by alternate elongation and contraction, performing also the office of suckers. Like most other zoophytes, they have the power of reproducing parts of their bodies which may have been accidentally destroyed.
Asterias, or Star-fishes.