CLASSIFICATION 46 1 



the aboral surface, or their distribution over the whole surface of 

 the body. What connexion, if any, the presence of these pave- 

 ment-like plates has with the habits it is impossible to say, but 

 it is unlikely to be of the high importance with which it was 

 regarded by Sladen, for in the same family we have genera with 

 inconspicuous marginals {Asterina) and others with conspicuous 

 marginals (Palmi2)es). The restriction of the papulae to the 

 back also varies within the same family (Linckiidae), and 

 whilst, on the whole, it is perhaps a primitive arrangement, 

 it is in many cases connected with burrowing habits, which 

 can scarcely be deemed to have been the original mode of life 

 of the class. 



A far better basis is supplied by the system of Perrier,^ 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. Spixulosa. — 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 com- 

 posed 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. JMarginals 

 large. Papulae only on dorsal surface. Tube-feet mostly devoid 

 of suckers. 



Order 4. Valvata. — Asteroidea in which the dorsal surface 



' lies. sci. Exi^td. Traraillenr et Talisman, " Ecliinodermes," 1894, pp. 10-15. 



