212 Prof. F. J. Bell on the Arrangement and 



free edge, but tliose of an Asterid or an Ophiurid by interca- 

 lation between the radial and the terminal. 



The Stelleridea then of earlier writers are marked by the 

 common character of being azygopodous ; for a time, no 

 doubt, the Stellerid descendant of the apelmatozoic Cystid 

 was neither distinctively Asteroid nor Ophiuroid ; and at this 

 stage I imagine one should place those fossil Stellerids whose 

 exact systematic position is a matter of such difficulty. 



Bnt it is no less clear that the Asteroid and Ophiuroid types 

 of organization are very different ; in one the organs of the 

 body have radial extensions, in the other there is a concen- 

 tration of the viscera comparable to that seen in the external 

 conformation of the body : in the one the radial extensions 

 are grooved beneath and the ambulacral ossicles are mere 

 serial repetitions, which remain as independent of their neigh- 

 bours as the nature of the case permits ; in the others there is 

 a marked tendency towards a solidification of the arm, the 

 ossicles are articulated to their neighbours, and the physio- 

 logical unity of each arm becomes marked. 



When looked at as a whole, and I may be permitted to 

 point out that it is long since the classes of Echinoderms 

 have been thus critically considered, the essential characters of 

 the groups of Echinoderms are seen to be somewhat different 

 from those which systematists have been in the habit of using. 

 It is in the hope that this general view has led to a more 

 correct appreciation than is possible when one class alone is 

 considered that I bring these generalizations and speculations 

 before those who are interested in the problem. 



Put in the ordinary linear way the proposed arrangement 

 will read thus : — 



Branch A. INCALICULATA. 



Stage a. Anactinogonidiata. 



Class. 1. HolotJiurioidea. 



Branch B. CALICULATA. 



Stage a. Anactinogonidiata. 



Class 2. Some Cystidea (?). 



Stage /3. ACTINOGONIDIATA. 

 1st Sub-branch. Statozoa. 



