220 ECHINODERMATA. 



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 (i.e., 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. The formation of the 

 calcareous foundation of the future adult is of the greatest 

 morphological importance and interest. Only in a very few 

 cases is the development direct. 



Following the excellent account of Echinoderm embryology, contained 

 in the Vergleichende Eniuiicklungsgeschichte der wirbellosen Thieren (Jena, 

 1890), by Korschelt and Heider, we shall distinguish four stages in 

 development ; — 



1. The formation of the primary germinal layers, of the mesen- 



chyme, and of the mouth and anus ; 



2. The origin of the enterocoele (body-cavity) and of the hydroccel 



(water-vascular system) ; 



3. The differentiation of the typical larval forms ; 



4. The modification of the larva into the adult Echinoderm. 



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

 and of the mouth and anus. 

 There is no difficulty in understanding how ectoderm and endoderm 

 are established by the invagination of a blastosphere or blastula. The 

 result is a ciliated gastrula. From the invaginating endoderm, somewhat 

 amoeboid cells are liberated into the persisting segmentation cavity. 

 These will afterwards form part of the middle stratum, and it is likely 

 that some of them remain unspeciahsed to form the rudiments of the 

 future reproductive organs. 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 stomatodreum. 



2. The formation of the enterocal (body-cavity) and the hydrocal 



[water-vascular sysief/i). 



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- 



