122 



PHYLUM ECHINODERMATA. 



apical system can sometimes be made out, but as a rule the plates 

 are indistinguishable from the other plates of the antambulacral 

 surface. In Holothurians the plates of the oral and apical 

 systems are absent both in the larva and in the adult. In 

 Echinoids, in which a central plate can often be made out, the 

 five interradial basals (genital plates) are always present, but 

 infrabasals and radials are not found. The oculars of Echinoids, 

 which are often called radials, are rather comparable to the 

 terminals, which must now be referred to. Whereas the plates 

 of the apical system, viz., the central, the infrabasals, the 



..Cj 



FIG. 84. Apical system of a young 

 Ophiurid Amphiura squamata (after 

 P. H. Carpenter), ba basal ; ce cen- 

 tral ; ib infrabasal ; r radial ; t ter- 

 minal. 



FIG. 85. Apical system of a Crinoid (Cyatho- 

 crinus). ba basal ; c2 second costal (primi- 

 brach) ; ib infrabasal ; ian anal interradial ; 

 r radial (from Lang). 



basals and the radials are developed round the right coelomic 

 vesicle of the larva, the terminals are five radially placed plates 

 on the oral wall of the left coelomic sac. They become the 

 oculars of Echinoids and those plates of Asteroids and Ophiu- 

 roids which are placed at the ends of the arms on the aboral side 

 of the projecting end of the radial water- vascular canal. 



The question as to whether thess plates are in all cases homologous is 

 a very difficult one to answer. It was originally suggested by Loven 

 and has been maintained by many of the later students of the group 

 notably by -P. H. Carpenter that there is a general homology 

 between these plates in the different classes ; but recently some doubt 

 has been thrown upon this view of them (see p. 292). It is possible 



