426 



EMBRYOLOGY 



Spatangoid, Hemiaster austrah's, the eggs of which develop 

 within the ovary, and then pass into a kind of brood-cavity, 

 which lies over the ambulacral furrow and is formed of 

 close-set and connivent spines. Here the young sea-urchins 

 undergo direct development. 



IV. THE METAMORPHOSIS OF THE LARVA INTO 

 THE ECHINODERM. 



The metamorphosis of the larva into the Echinoderm takes 

 place most simply in the Holothurioidea, and therefoi-e we 

 consider this group first. 



Fig. 205. — A and B, metamorphosis nf the Auricularia larva of Synapta ^ligit^lta 

 into the pupal form (after Semox). a, anus; cd, proctodoeum ; cut, enteroccele ; 

 ki; calcareous wheels; m, oral funnel; mg, stomach; 11, nerve bands; u', water- 

 vascular ring with the evaginations (tentacular and radial vessels). 



Holothurioidea. — The metamorphosis of the Auricularia 

 into the Halothurian is manifested in the external shape of 

 the larva by the gradual disappearance of its lobular pro- 

 cesses and the alteration of the ciliated band, which breaks 

 up into several pieces (Fig. 205 A and B). The larger 

 number of these pieces alter their positions by acquiring 

 a transverse position in place of a longitudinal one (Fig. 

 205 B). At the same time the protuberances of the larval 

 body disappear, and it assumes more of a cylindrical form, 

 whei-eby, according to Sicmon, its circumference is strikingly 

 diminished. Finally the different pieces of the ciliated 



