I. ASTEROIDEA. 



333 



the ccelom and ambulacral system. This becomes divided, and one por- 

 tion develops itself as a ring around the oesophagus, the future ring 

 canal, and from this five outgrowths, the radial canals, arise. Since these 

 canals, as they grow out, carry the body walls before them, the arms in the 

 starfishes, which show the process most clearly, arise as outgrowths which 

 recall buds (fig. 308). This has given rise to one view which regards the 

 arms as individuals, the whole body (and hence that of all echinoderms) as 

 a colony of five individuals. According to this the development would be 

 a kind of alternation of generations, the larva being the asexual genera- 



FIG. 307. 



FIG. 308. 



FIG. 307. Formation of the coelom in Echinus. (From Korschelt and Heider.) A, 

 first anlage of coelom; B, later stage; C, complete constriction of coelom (vaso- 

 peritoneal vesicle) from archenteron 



FIG. 308. Formation of Ophiuran from the pluteus larva. (After Miiller, from Kor- 

 shelt-Heider.) 



tion which by budding produces the colony. Yet this view does not agree 

 with the actual relations, since it draws an untenable contrast between the 

 larva and the perfect echinoderm. The most important organs of the 

 former are carried over into the latter, and the echinoderm brings the anla- 

 gen to further development. In the insects many features which are lack- 

 ing or incompletely developed in the larva are developed in the course of 

 the metamorphosis. There is a metamorphosis in the echinoderms as in 

 insects. It is a question as to which group of Echinoderma is the most 

 primitive, but ease of treatment makes it best to begin with the Asteroidea. 



Class I. Asteroidea (Starfish). 



Two parts can' be recognized in the body of a starfish, a 

 central disc and the arms, usually five in number, which radiate 

 from it (fig. 316). The relations in which these parts stand to 

 each other vary between two extremes. In many starfish the 

 arms play the chief role and the disc appears as only their united 

 proximal ends (figs. 309, 310). On the other hand the disc may 



