A far better basis is supplied by the system of Perrier,[[455]] who divides the Asteroidea into five orders according to the character of the dorsal skeleton; and this classification really corresponds with the different habits assumed by groups of Asteroidea in order to meet what must be regarded as one of their chief dangers, viz. assaults by other animals, especially parasites, on their soft and delicate skins. Since the food (so far as is known) of all Asteroidea is more or less similar, the great differentiating factor in their development must have been the means they adopt to shelter themselves from their enemies. Perrier's classification, which we shall adopt, is as follows:—

Order 1. Spinulosa.—Asteroidea in which the plates of the dorsal skeleton bear spines arranged singly or in groups. The tube-feet have suckers and there are no pedicellariae. Marginals sometimes conspicuous, sometimes rod-like.

Order 2. Velata.—Asteroidea in which the dorsal surface of the animal is concealed from view by a false membrane composed of the webs of skin stretched between diverging groups of spines united at the base with one another. No pedicellariae. Tube-feet with suckers.

Order 3. Paxillosa.—Asteroidea in which the dorsal surface is beset with paxillae (upright spines bearing two or three circles of horizontal spinelets). Pedicellariae, when present, few, and never of the forcipulate variety; often absent. Marginals large. Papulae only on dorsal surface. Tube-feet mostly devoid of suckers.

Order 4. Valvata.—Asteroidea in which the dorsal surface is protected by plates covered with a mail of minute granules. Pedicellariae of the valvate or alveolate type. Marginals large.

Order 5. Forcipulata.—Asteroidea in which the dorsal surface is beset with small spines surrounded by numerous forcipulate pedicellariae. Tube-feet with suckers and arranged in four rows. Marginals rod-like and inconspicuous.

Order I. Spinulosa.

This is by far the most primitive order of Asteroidea. The tube-feet are arranged in two rows only, and there is no special means of protecting the back, other than the small close-set plates bearing spines, with which it is covered. In some cases, as Asterina, these spines have a tendency to converge when irritated, and thus act somewhat like pedicellariae. This circumstance suggests strongly the manner in which pedicellariae have been developed from small groups of spines. The order is divided into six families, of which four have common representatives on the British coast.

Fam. 1. Echinasteridae.—Spinulosa in which the aboral skeleton is composed of close set plates bearing comparatively small spines. This family is represented on the British coasts by the beautiful scarlet Starfish Cribrella (Henricia) sanguinolenta. It is also found on the Norwegian coast and on the east coast of North America. On the Pacific coast it is replaced by a larger species, C. laeviuscula. The narrow ambulacral grooves and sluggish movements at once distinguish it from the Starfish described as the type. Indeed, all the Spinulosa seem to be slow in their movements in contrast to the comparatively active Asterias and its allies. Cribrella is remarkable for its large eggs, which have a rapid development. The larva never swims at the surface but glides only for a short time over the bottom. Echinaster is an allied genus in which each plate bears a single somewhat enlarged spine. It possesses on the skin of the aboral surface numerous pits lined by glandular walls, which probably secrete a poisonous fluid which defends it. Acanthaster has thorny spines, more than ten arms, and several stone-canals and madreporites.

Fam. 2. Solasteridae.—Spinulosa in which the aboral skeleton is a network of rods. Spines arranged in diverging bundles (sheaves) attached to a basal button. This family includes the well-known "Sun-stars," with numerous arms and a wide peristome. There are two species found on both sides of the Atlantic. Solaster papposus, with thirteen or fourteen arms and long bundles of spines on the dorsal surface, which is of an orange colour variegated with yellow, and S. endeca with eleven rays and shorter spines and of a reddish violet colour. Rhipidaster has eight arms. Some genera have, however, only five arms, as, for instance, Peribolaster and Korethraster (Fig. 197). In this family there are conspicuous "Polian vesicles" attached to the water-vascular ring.