132 Introduction to A nimal JMorphology. 



with halves more separable than in the succeeding joints, and 

 they are joined to their fellows of the other limbs so as to 

 form a calcareous ring, enclosing the water-vascular circlet, 

 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 are four ; one, which in Asteriadce is anterior to the 

 wall with the stone canal, being absent. Between the arms are 

 genital fissures, through which the products of the inter- 

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

 stomach is round or pentagonal, ciliated ; the anus absent. 

 A few are viviparous, and rapidly developed (Ophiolepis, 

 squamata, and vivipara), but usually there is a free larval stage 

 (Fig. 19). The ciliated ovum assumes a bilateral form, like a 

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

 two lateral primary rays, a calcareous spicule {s), at whose base 

 is a half circle also calcareous ; from these diverge respect- 

 ively two lower and two upper accessory rays, the latter of 

 which bifurcate ; thus there are eight diverging calcareous 

 rods over which the homogeneous protoplasm is spread. 

 Two little spines lie fore and aft where the calcareous basal 

 semicircles unite. These spicules are in contact, but not 

 ankylosed together. A central digestive canal 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 phary-nx ; then a cup-like 

 mass forms behind it ; these represent respectively the ventral 

 and 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. Euryalidae — 



