thl: development of opuroTHiux fiuculiis. 595 



be missed out, tliis will be done. So Ave explain the missing 

 out of the Mysis stage in the embr3'ology of the crab, for 

 instance, aud the passing over numerous larval stages within 

 the egg-shell in other cases. The smallness of cavities and 

 thickness of the tissues derived from their walls is a general 

 feature of Ophiuroidea which runs throughout the entire class, 

 and is an adult feature retrospectively affecting development. 

 The epineural folds require, of course, no comment, as 

 they are one of the features in which Ophiuroidea have 

 progressed beyond Asteroidea. They made their appearance 

 in ontogeny rather earlier than they should, and in so 

 far are another example of adult features affecting the 

 ontogen3^ The retention of the larval mouth, however, 

 cannot be viewed otherwise than as a primitive feature. 

 We cannot imagine that in the history of the race a period 

 intervened when the previously existing mouth disappeared 

 and a totally now one made its appearance, but why the 

 mouth should persist in Ophiuroidea and Holothuroidea and 

 be lost in Asteroidea and Echiuoidea is a mystery. We may 

 notice, however, that the mouth is retained in Crinoidea also, 

 for though the larval stomoda3um, as Bury calls it (4), never 

 opens into the yolky, fuuctionless gut, yet there is no doubt 

 from its position of its homology with the stomoiheum of 

 other Echinoderm larvte. Now this stomodasum is shifted 

 postei'iorly and opens out to form the vestibule of the fixed 

 larva, out of which the tentacles project. Asimilar fate befalls 

 the stomoda^um in Ophiuroidea. Owing to the shrinkage 

 of the forehead it opens out, and the primary tentacles 

 of the hydroccele project through it. The same thing happens 

 in the development of Holothuroidea. A difficulty may here 

 occur in the minds of some that, in the development of 

 Synapta digitata, the only Holothurid of which the whole 

 ontogeny is known, the first tentacles to make their appear- 

 ance are the buccal tentacles, not those terminating the radial 

 canals. But from Metschnikoff's paper (21) it appears that as 

 the radial canals grow back, extensions of the "atrium '^ (as 

 the stomodaeum is called), accompany them, and hence it may 



