ECIIINODERMATA. 

 Fig. 16. 



37 



Asterias rubens : three rays 

 d, d, a 



in the one, at A, caeca cut short to shew the vesicles of the feet, 

 one ovary, o; g, g, caeca, g' g', coeca inflated. 



into it from above, which are probably secre- 

 ting organs. The coeca are thin and mem- 

 branous like the stomach; each consists of a 

 central tube with lateral branches, which in 

 their turn are lobed or branched, and terminate 

 in cellular dilatations. The two cosca of a ray 

 sometimes communicate with the stomach by a 

 short single tube (/?); in other cases they have 

 separate orifices. They do not reach so far as 

 the distal end of the ray; each one is attached 

 to the roof by what might be called a double 

 mesentery, for the peritoneum forms here two 

 duplicatures (figs. 12 and 16, TZ,) between the 

 coecum and the roof of the ray. A space is 

 inclosed between these duplicatures which 



opens into the central part of the body at the 

 root of the coeca. 



Such is the structure in the Asterias, but in 

 some other genera belonging to the tribe of 

 Asteroidea it is different. In Ophiura, Eu- 

 ryale, and Comatula, in which the rays are 

 very long and slender, the cceca are mere cel- 

 lular dilatations of the stomach, and do not 

 extend into the rays. Comatula moreover dif- 

 fers from all the tribe, inasmuch as its alimen- 

 tary canal has two openings, a mouth and anus, 

 situated near to each other on the ventral sur- 

 face. 



The mouth of the star-fish is very dilatable, 

 so as to admit large mollusca in their entire 



