248 ECHINODERMA. 



The larva is, first of all, a slightly modified, diffusely 

 ciliated gastrula. It becomes more modified, but preserves 

 a bilateral symmetry. In Holothuroids, Echinoids, Asteroids, 

 and Ophiuroids, the larva becomes quaintly modified by the 

 outgrowth of external processes, and the formation of special 

 ciliated bands. The larva of Crinoids (/.., of Antedon only) 

 is not so divergent. 



The larva does not grow directly into the adult. On the 

 contrary the adult arises, for the most part, from new growth 

 within the larva. The structures peculiar to the larva are 

 absorbed, or in part thrown off. Only in a very few cases 

 is the development direct. 



Following the excellent account of Echinoderm development, in the 

 Vergleichende Entwicklungsgeschichte der wirbellosen Thieren (Jena, 

 1890), by Korschelt and Heider, we distinguish four stages : 



I. The formation of the primary germinal layers* of the mesenchyme, 

 and of the mouth and anus. 



Ectoderm and endoderm are established by the invagination of the 

 blastosphere. The result is a ciliated gastrula. From the invaginat- 

 ing endoderm, somewhat amoeboid cells are liberated into the persisting 

 segmentation cavity, and form the mesenchyme tissue alluded to above. 

 The gastrula cavity or archenteron is the larval mid gut ; the blastopore, 

 or mouth of the gastrula, seems usually to become the anus ; but an 

 invagination taking place at the other end forms a short fore gut or 

 stomatodseum. 



2. The formation of the enteroccel (body cavity] and the hydroccel 

 (water vascular system). 



There is a close connection between the origin of the body cavity and 

 that of the water vascular system. Both are the results of an outgrowth 

 or of outgrowths from the gastrula cavity or archenteron, into the sur- 

 rounding space between endoderm and ectoderm. As they have a 

 common origin, the outgrowth or outgrowths which give rise to enteroccel 

 and hydroccel may be termed vaso-peritoneal. 



There is not perfect agreement as to this united origin, but the follow- 

 ing facts are generally recognised. 



In Holothuroids there is a single outgrowth which gives rise to 



both body cavity and water vascular system. 



In Echinoids, Asteroids, and Ophiuroids, there are two out- 

 growths, from the left of which the water vascular system 

 arises. 



In Crinoids (Antedon}, there are three outgrowths, that which 

 gives rise to the water vascular system being independent of 

 the pair which form the body cavity. 



In most cases a dorsal pore bringing the hydroccel into com- 

 munication with the exterior has been detected. 



