160 PHYLUM ECHINODEKMATA. 



(Fig. 116, c) by the formation of the arms, which grow out from 

 the sides of the body aborally to the water-pore and anus and 

 between the oral and basal plates. The result of their out- 

 growth is that the oral surface of the body is much enlarged by 

 the formation of the tegmen calycis beyond the circle of the oral 

 plates which become reduced in size and eventually disappear. 

 On the aboral side of the calyx, calcareous plates, the radials, are 

 formed to support the growing arms. Finally the calyx be- 

 comes detached from the stalk, and the free adult state is reached. 



The Crinoids, in so far as relates to the development of the 

 coelom, differ from all other Echinoderms in the fact that the 

 rudiment of the posterior coeloms is budded off from the gut 

 independently of the anterior coelom and hydrocoel. They 

 resemble certain of the Holothurians in the fact that the anterior 

 coelom becomes merged in the general body cavity, but differ 

 from these in the retention of the water -pore. In becoming 

 attached by the preoral lobe they resemble the larvae of certain 

 Asterids, but they differ, as already explained (p. 154), from these 

 in the relation which the attaching surface bears to the adult 

 structure. 



Affinities. The fundamental fact in the morphology of the 

 Echinoderms is the enterocoelic origin of the coelom. In this, 

 as has been already pointed out in the second volume of this 

 work (chap, i., p. 7) they are associated with the Brachiopoda 

 Chaetognatha, Chordata and probably the Phoronidea. Whether 

 we are to regard this fact as indicating affinity it is difficult 

 to say. In the absence of evidence tending to unite any of 

 these groups more closely or as closely with any other group 

 of the animal kingdom, we may perhaps consider this common 

 feature as sufficient justification for treating them in immediate 

 succession to one another, but we must not attribute too much 

 importance to it, for it is absent from both vertebrate and 

 tunicate development, nor is it found in Annelids, Arthropods 

 and Molluscs, the coelom of which is clearly the homologue of 

 the coelom of enterocoelic forms. The question now presents 

 itself, do the Echinodermata possess any features which enable 

 us to associate them more closely with any particular phylum of 

 the Enterocoela than with the others ? It has been pointed out 

 by some zoologists, amongst whom I may specially mention 

 Mao Bride, that in the primitive disposition of their coelomic 



