THE ECHINOIDEA 



325 



Fio. XLVII. 



larval form of Hemiaster cavernosa, in the endocyclic 

 stage. (After Agassiz.) 



Each of the main groups of Echinoderma has been at one time 

 regarded as the ancestor or nearest ally of the Echinoids, and the 

 question is still highly conjectural. Embryology gives very little 

 assistance. Study of the development of a young Echinothurid, 

 Echinocyamus, or Hemiaster (Fig. XLVII.) teaches important lessons 

 as to the affinities between those forms and other Echinoids. It 

 shows that the young Echinothurid resembles the Diademoida, 

 and that the young Hemiaster is endocyclic. But the earlier larval 

 stages have been so affected 

 by secondary variations 

 that they give no satis- 

 factory information as to 

 whether the Echinoids are 

 nearest to the Cystids, 

 Crinoids, Holothurians, or 

 Stelleroids. 



The Crinoids are so 

 unlike the Echinoids in ap- 

 pearance and structure, 

 that we know of no form 

 that appears to link the 

 two classes. Neverthe- 

 less, the Echinoidea have been regarded as descended from a 

 Crinoid-like ancestor. The acquirement of a radial symmetry 

 was unquestionably the most important event in the develop- 

 ment of the ancestral Echinoderm ; it is easiest to explain this 

 as the result of fixation, and therefore the fixed, stalked forms 

 have been claimed as the ancestors of the free forms. It is further 

 argued that this conclusion is supported by the occurrence on the 

 abactinal side of some Echinoids and Stelleroids of a series of plates 

 known as the apical system. This system includes a central plate 

 surrounded by two circles of plates. The theory has been urged, 

 especially by the late P. H. Carpenter and by W. P. Sladen, that 

 the plates of this apical system are homologous with those of the 

 calyx of the Pelmatozoa, and are to be regarded as relics from a 

 period when these plates were of great functional importance. 

 Unfortunately for this view, however, the calycinal or apical plates 

 are either absent or unimportant in the oldest Echinoids and 

 Asteroids ; and it is in later groups, such as the Saleniidae and 

 Cidaridae, that the plates are developed on the supposed ancestral 

 plan. Moreover, instead of Tiarecliinus in which the apical plates 

 are most important being ancestral, it is almost certainly an 

 aberrant, and somewhat degenerate offshoot. 



The last blow to the idea of the apical plates of Echinoids 

 being homologous Avith the calyx plates of Crinoids, has been 

 given by MacBride, who, on embryological grounds, urges that 



