XVI 



ECHINODEKMATA 



543 



sacs, wliilst that of Auricularia is. filled with inesenchyme. The 

 mode of segmentation of the coeloni is widely different in Auricularia 

 from what it is in Bipiunaria. Ou the other hand, the retention of 

 the larval mouth and its shift towards the left, as seen in Holo- 

 thuroidea, is not found in Asteroidea or Echinoidea, and is paralleled 

 only hy the development of the Ophiuroidea. 



The growth of the buccal tentacles into the atrium, i.e. the larval 

 stomodaeum, recalls what occurs in Ophiuroidea and in Echinoidea, 

 if our conception of the amniotic space of the latter as a separated 



v 



cole 



calc 



FIG. 402. Post-larval stage in the development of Cucumaria x<u'i<:f>l<i viewed 



from the side. (Original.) 



b.t, buccal tentacles ; (./, paired tube feet ; t; valves into which the atrium splits. 



portion of the stomodaeum be admitted ; hut there is an important 

 difference, for the primary tentacles of Asteroidea, Ophiuroidea, and 

 Echinoidea are the tips of the radial canals, but the tips of the radial 

 canals of Holothuroidea do not protrude into the atrium, only their 

 basal branches, the buccal tentacles, do so. More light on this interest- 

 ing difference will be obtained when the complete history of Cucumaria 

 saxicola is worked out. This species, which is very closely allied 

 to Cucumaria planci, has been reared through the metamorphosis 

 by the authorities of the Plymouth Biological Station. 



The adult anatomy of a Holothuroid suggests the view that the 

 group may have been derived from early Echiuoidea in which the test 



