568 



ECHINOID PLUTEUS. 



form what is obviously a young Echinus, the arms of the Pluteus with their 

 contained skeleton have been gradually undergoing atrophy. They become 

 irregular in form, their contained skeleton breaks up into small pieces, and 

 they are gradually absorbed. 



The water-vascular ring is from the first complete, so that, as in 

 Asterias, it is perforated through the centre by a new oesophagus. According 



FIG. 266. SIDE AND DORSAL VIEW OF A LABVA OF STRONGYLOCENTRUS. 



(From Agassiz.) General reference letters as in figs. 264 and 265. 

 e". anterolateral arms; v" . ciliated epaulettes; -'. invagination to form the disc 

 of Echinus. 



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 epidermis 

 of the amnion, which is ruptured in the process and eventually thrown 

 off. 



Crinoidea. The larva of Antedon, while still within the 

 egg-shell, assumes an oval form and uniform ciliation. Before it 



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

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

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

 Ophiuroids. 



