426 



EMBRYOLOGY 



Spatangoid, Hemiaster australis, the eggs of which develop 

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

 which lies over the ambulacra! furrow and is formed of 

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

 undergo direct development. 



IV. THE METAMORPHOSIS OF THE LARVA 

 THE ECHINODERM. 



INTO 



The metamorphosis of the larva into the Echinoderm takes 

 place most simply in the Holothurioidea, and therefore we 

 consider this group first. 



Fig. 205.—^ and B, metamorphosis of the Auricularia larva of Synapta dlgit>tta 

 into the pupal form (after Semon). a, anus; ed, proctodaeum ; eat, enterocoele ; 

 kr, calcareous wheels; m, oral funnel; mg, stomach; n, nerve bands; u', water- 

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



Holothurioidea. — The metamorphosis of the Auricularia 

 into the Holothurian 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, 

 whereby, according to Semon, its circumference is strikingly 

 diminished. Finally the different pieces of the ciliated 



