6l2 



ECHINODERMATA 



formed about the same time. The primary pore -canal in 

 Asterina as in all Dipleurulae opens into the anterior coelom ; 

 the stone-canal is formed from a ciliated groove running along 

 the neck of communication between this and the hydrocoel/ 

 The constriction dividing the body into disc and stalk divides 

 the anterior coelom (single in Asterina as in the older Bipinnaria) 

 into two parts ; the portion included in the disc forms the axial 

 sinus of the adult. The lower end of the axial sinus expands and 

 surrounds the adult mouth, forming the inner perihaemal ring ; the 

 outer perihaemal ring is formed by the juxtaposition of four wedge- 

 shaped outgrowths from the left posterior coelom and one from 

 the anterior coelom. From these the radial perihaemal canals 



ABC 



3 n 



Fig. 289. Views of larvae of Asterina gibbosa in the course of metamorphosis. A, larva 

 of eight days, from the right ; B, left, and C, right view of larva of nine days ; 

 1-5, lobes of hydrocoel ; I-V, rudiments of arms. (After Ludwig.) x 45. 



subsequently grow out into the arms. The metamorphosis of 

 Bipinnaria has been well worked out by Goto, 1 and it agrees in 

 essential features with that of Asterina gibbosa; in fact, the 

 differences which Goto maintains between the two types may 

 be reasonably explained on the supposition of some stages 

 having escaped the notice of this observer. The larva develops 

 on the apex of the prae-oral lobe three papillae for occasional 

 attachment,' 2 and in the centre of these a cup-shaped disc for 

 permanent fixation when the prae-oral lobe is converted into a 

 stalk. When these papillae (Fig. 290, fix) have been developed 

 the larva is known as a Brachiolaria. 



1 "The Fate of the Body-cavities in the Metamorphosis of Asterias pallida ," J. 

 Coll. Japan, x. 1898, p. 239. 



2 Yves Delage, "Elevage des Larves parthenogenetiques a" Asterias glacialis," 

 Arch. Zool. Exp. (4) ii. 1904, p. 27. 



