Echinodermata (spiny skinned) have all their parts symmetrically arranged about a central axis, a contractile heart, good digestive organs, and a peculiar system of radiating canals extending through the organism. They are a numerous family of exclusively marine animals, and their characteristics furnish an interesting study. Most naturalists mention four classes.
Class I.—Crinoidea, or sea lilies, now nearly extinct, are fixed to rocks, or the sea bottom, by what seems simply a stem, but is the body of the animal. At the top is the mouth, resembling an expanding bud or flower that opens upward, surrounded by long tentacles, or arms, not unlike the sea anemone. It is supported by a calcareous skeleton consisting of many members, stiff-jointed, crossing and interlacing with one another. When living, the gelatinous animal partly envelops this framework.
Class II.—Asteroidea, or star fish, have a flat disc, five or more radiating arms extending some distance from the body at the center, and containing a part of the viscera. The mouth is where the arms meet, and opens downward. The upper surface is studded over with rough knobs, between which are the openings of many little tubes, for the passage of water into and out of the body. The round mouth is very dilatable, enabling it to receive large and solid objects for its food, which there is no attempt to masticate. After the digestive organ is somehow wrapped around the shell fish on which it feeds, it is held in its firm embrace till the nutritive portion is disposed of, and then thrown out. They are voracious eaters, devouring all kinds of garbage that would otherwise accumulate along the shores, valuable as sea scavengers, though destructive of living crustaceans and shell fish.
Class III.—Echinoidea (hedgehog-like) are covered with spines which they move either by the enveloping membrane or by small muscles properly situated for the purpose. The thin, horny, and, when dry, very light skin is peculiar, in that it is composed of regularly shaped, pentagonal plates, arranged in radiating zones, every alternate plate perforated with small holes; and among the spines, but capable of extending beyond them, are little arms, provided at the end with forceps, probably for seizing their prey, or for ridding themselves of troublesome parasites. These are also used for locomotion. They are less active than some others of the family, live near the shore among rocks, or under the seaweed, feed on crabs, and are oviparous.[3]
Class IV.—Holothuroidea (whole mouthed). They are elongated, like a cucumber, and the head end terminates abruptly, the mouth being a circular opening surrounded with feathery tentacles. They have remarkable muscular power, by which they can disgorge the contents of the stomach, throw off their tentacles, and even eject most of their internal organs, and survive the loss, afterward producing others, perhaps more satisfactory or in a healthier condition. In tropical climates, they have been likened to the sea urchin, without a shell, but are proportionally longer, and their axis horizontal.
SUB-KINGDOM V.
Vermes (worms). Animals having head and tail composed of segments. The digestive organ is tubular, and the nervous system a double chain of ganglia[4] on the ventral[5] surface. There are six classes of vermes. The animals differ greatly in appearance.
Class I.—Flat worms are best known as the parasites that infest animals, such as the liver fluke of the sheep, and the tapeworm. The flat worms pass through a very peculiar metamorphosis, some varieties taking as many as seven different forms.
Class II.—Round or Thread Worms are represented by the pin worm and Trichina. The latter is the dangerous worm which finds its way into the human system from pork flesh, in which it is imbedded.
Class III.—Wheel Animalculæ, or Rotifera. A most interesting microscopic worm, abounding in fresh water and in the ocean. They will remain dried up for years, and then recover life. Their shapes are very peculiar.