/. ASTEROIDEA. 



333 



the coelom and ambulaoral sj'stem. This becomes divided, and one por- 

 tion develops itself as a ring around the cesophagus, 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 whicli 

 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 geuera- 



Fio. 307. 



Fig. 308. 



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

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

 peritoneal vesicle) from archenteron 



Fig. 30K.— (""ormation of Ophiuran from the pluteus larva. (After MuUer, from Kor- 

 shelt-Heider.) 



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

 with the actual I'elations, since it draws an untenable contrast between the 

 larva and tlie perfect ecliinoderm. The most important organs of tlie 

 former are carried over into the latter, and the echinoderm l)rings the anla- 

 gen to further develo]jment. In the insects many features which are lack- 

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

 the metamorphosis. Tliere is a metamorphosis in tlie 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 tlie body of a starfish, a 

 central disc and the arms, nsuall)' five in number, which radiate 

 from it (fig. 310). Tlie relations in whicli these parts stand to 

 each other vary between two extremes. In many starfish the 

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

 jiroximal ends (figs. 300, 310). On the other hand tlie disc may 



