viii ECHINODERMATA ONTOGENY 517 



The water pore lying on the right side of the mesentery (now formed) disappears 



19 



FIG. 415. Longitudinal section of a larva of Cucumaria doliolum (aftp r Selenka). 1, Frontal 

 prominence with enclosed jelly-like substance; -2, tentacle vessels; 3, 6, 15, 14, 13, radial 

 vessels ; 5, stone canal ; 8, madreporite ; 4, Polian vesicle cut off ; 7 and 12, coelom, enterocoel 

 vesicle ; 9, anus ; 10 and 11, the first two tube-feet ; 16, circular canal of the water vascular 

 system ; 17, oesophagus ; 18, mouth ; 19, mesenchyme cells ; II, III, IV, V, ciliated rings. 



later, and still later the madreporitic vesicle opens into the body cavity, and thus 

 becomes the secondary inner madreporite. 



