\ 



VIII. Comparison with Structure of Asterozoa. 1915 322 



pp. 115-18, pis. xiv-xvi), it is natural to suppose that the ciliary 

 and podial method was that chiefly practised by those earlier Asterozoa 

 in which the mouth-frame was still but slightly differentiated. 



We have now reached this stage of our discussion : we have seen, 

 on the one hand, that by the middle of the Ordovician Epoch certain 

 Pelmatozoa had acquired a structure of the rays and of the peristome 

 strangely like that of the Asteroidea; and, on the other hand, that 

 the Asteroidea and, to a less extent, the Ophiuroidea, as they are 

 traced back through the geological periods, approach more and more 

 in the same respect to the structure of the Edrioasteridae. The 

 conclusion that suggests itself is obvious, but not necessarily correct. 

 Many difficulties have to be smoothed away before a genetic filiation 

 between the two groups can be maintained. 



The statement, for instance, that Asteroidea occur in the Cambrian 

 is frequently made, and is always quoted by those who regard the 

 Starfish as a very primitive group, or who deny the possibility of 

 their descent from a form similar to any Edrioasteroid. So long as 

 the term Cambrian was extended to the summit of the Caradocian, 

 the statement was admissible. Nowadays it is misleading. No 

 Asterozoa are yet known below the Middle Ordovician ; Edrioasteroidea 

 are known from the Middle Cambrian, and typical Edrioasteridae were 

 contemporaries of the earliest known Starfish. 



Other difficulties are presented by differences of structure and by 

 certain features of the development. These will form the subject of 

 the next Study. 



