ASYMMETRY IX THE STARFISH. 115 



may be passed over, at least until more advanced stages are 

 reached. The events associated with the formation of the system 

 of coelomic pouches, however, need more detailed treatment for 

 they seem to be unique in certain respects. 



Development of the Ccelomic Structures. As has already been 

 said, the first change leading to the formation of coelomic struc- 

 tures is the enlargement and thinning out of the free end of the 

 archenteron. This is well shown in Fig. 5. The next stage 

 involves a flattening out mushroom-fashion of this thin-walled 

 vesicle, as shown in Fig. 6. Here the paired anterior coeloms are 

 shown in process of being pinched off. This process completes 

 itself, as shown in Fig. 7, by the end of the third day. I have 



1. a.c... 



l.p.c.- 



f.a.c. 



7 



FIGS. 7-9. Early Bipennaria larvae, showing especially the formation of 

 ccelomic structures, a.c., anterior coelom; c.b., ciliated band in optical section; 

 h.p., hydropore; 1. a.c., left anterior coelom; l.p.c., left posterior ccelom; r.a.c. t 

 right anterior coelom. 



never been able to assure myself, as other writers have done, that 

 the left anterior coelom is from the first larger than the right. It 

 appears to me that until the formation of the posterior coelom the 

 larva is perfectly bilaterally symmetrical. 



The Posterior Ccelom. A very significant event, from the stand- 

 point of the development of asymmetry, is the appearance of the 

 posterior ccelomic vesicle. In Patiria, this vesicle is a conspicu- 

 ous object for a considerable period of time and in the end becomes 

 the hypogastric ccelom. This condition is like that described for 

 Cribrella and Solaster and unlike that in most asteroids thus far 

 described in which the hypogastric ccelom is derived from a 



