vm EGHINODERMATA MORPHOLOGY OF SKELETON 327 



be infrabasals. This view is, however, not certain, because other 

 new and also radially arranged plates may be added to these, which 

 may thus also themselves possibly be accessory structures. 



III. Ophiuroidea. 



In this class, the plates of the apical system do not appear in the 

 embryo in exactly the same order as in the Asteroidea. First the five 

 radials and the central plate form, and, somewhat later, between the circle 

 of radials and the central plate, the five basals and the five infrabasals 

 appear. In many Ophiuroidea, an embryonic condition of the apical 

 system is retained in the adult, the central plate being surrounded by 

 the circle of five radials, while the basals and infrabasals are wanting 



- -b 



FIG. 287. Plates of the apical system of the disc of 

 Ophiomusium validum (after P. H. Carpenter). For 



lettering see p. 317. 



FIG. 288. Apical system of a 

 young Amphiura squamata (after 

 P. H. Carpenter). For lettering 

 see p. 317. 



(species of the genera Ophioglypha, Ophiomastix, Ophiopyrgus, Ophiura, 

 Hemipholis, Ophioceramis, OphiopJiolis, Ophiotrochus). In many others, 

 however, there are, besides the radials, the five basals, which may 

 vary greatly in size (species of the genera Ophioglypha, Ophiomastix, 

 Ophiomusium, Ophiura, Ophiopholis, Ophiozona, Ophiactis, Ophiolepis). 

 In Ophiomitra exigua there is only the central plate with five basals 

 around it. In some Ophiuroidea a complete apical system is developed, 

 infrabasals being added to the basals, the radials and the central 

 plate (isolated species of Ophioceramis, Ophioglypha, Ophiozona, Ophio- 

 musium (Fig. 287), Ophiolepis). In very many Ophiuroidea the 

 calcareous plates developed at the apical surface of the disc are so 

 numerous that it is then impossible to recognise among them the 

 typical plates of the apical system. The adult Ophiuroidea have no 



