132 Introduction to Animal j\ [or pliology. 



with halves more separable than in the succeeding joint 

 they are joined to their fellows of the other limbs s<> 

 form a calcareous ring, enclosing the water-vascular t -in-let, 

 whose ten branches to the oral tentacles pierce these plates. 

 These hollow tentacles are in pairs at each angle of the 

 mouth, one on each side of the radial water-vessel. The 

 madreporiform plate is ventral, distinct, or more commonly 

 covered by one of the oral plates. The stone-canal some- 

 times contains a free or attached pulpy mass. The, Polian 

 vesicles ar'e four; one, which in Asteriadie is anterior to the 

 wall with the stone canal, being absent. IVlween the arms an; 

 genital fissures, through which the products of the inter- 

 radial sexual glands escape, and the sea-water enters. The 

 his round or pentagonal, ciliated; the anus absent. 

 A few are viviparous and rapidly developed ( )phi< 

 sijuamata, anil vivipara', but usually there is a free larval 

 (Fig- in). The ciliated ovum assumes a bilateral form, like a 

 compressed eight-ribbed umbrelia, which has, in each of its 

 two lateral primary rays a calcareous spicule \5\ at whos< 

 is a half circle al-o call Tom tlu-se diver 



ively two lower and two upper ; rays, the lav 



which bifurcate: thus then- ! it diverging calcareous 



which the homogeneous protoplasm is sj 

 Two little spines lie fore and aft where the calcar 

 semicircles unite. The-e spicules are in contact. In: 1 

 ankylosed together. A central di anal forms (/), ap- 



pearing deep green from its contents ; traces of the nerve 

 cords next appear. The plasma around the stomach forms 

 two longitudinal lateral folds, which unite as a lamina in front 

 of the stomach, traversed by the pharynx; then a cup-like 

 mass forms behind it; these repre>ciit respectively the ventral 

 ami dorsal faces of the perisome. The adult rays soon 

 appear, grow, and become shielded. Then the lobate body 

 of the larva breaks up, and atrophies, and, together with 

 the skeleton, is lost, and the adult form is assumed. The 

 free-swimming, pelagic larva of Ophiolepis ciliata is known 

 as Pluteus paradoxus, that of O. Sundevallii as P. bimacu- 

 latus. 



They are divisible into two families: i. Euryalidx 



