474 



ECHINOID PLUTEUS. 



to Agassiz the first five tentacles or tube feet grow into the radial canals, 

 and form the odd terminal tentacles exactly as in Asterias 1 . Spatangus 

 only differs in development from Echinus in the fact that the opening 

 of the invagination to form the ventral disc becomes completely closed, and 

 that the tube feet have eventually to force their way through the larval 



FIG. 267. FULL-GROWN LAEVA OF STBONGYLOCENTRUS. (From Agassiz.) 



The figure shews the largely-developed abactinal disc of the young Echinus 

 enclosing the larval stomach. Keference letters as in previous figs. 



1 Gotte (No. 549) supported by Mliller's and Krohn's older, and in some points ex- 

 tremely erroneous observations, has enunciated the view that the radial canals in 

 Echiuoids and Holothuroids have a different nature from those in Asteroids and 

 Ophiuroids. 



