U BRITISH PALEOZOIC ASTEROZOA. 



C. THE ACCESSORY OSSICLES OF THE DISC. 



The Apical Sijstem of Aateroidea and Ophiiiroidea. — In certain genera of Asteroi- 

 dea and of Ophiuroidea the apical snrface of the disc is covered with plates which 

 are arranged in the symmetrical manner diagrammatised Text-fig. 20. The ari'ange- 

 ment of these plates recalls the disposition of the calyx plates in the Pelmatozoa, 

 and varions anthors, '^.(/.Carpenter and Loven, have, in consequence, attached 

 great importance to it as suggesting the common ancestry of the f'rinoidea, the 

 Asteroidea, and the 0])hinroidea (the so-called " calycinal " theory). 



0^''(? O ©■■■■■.■A 



IB 



Text-fig. 29. — Diagrammatic representation of disc of an Asteroid to illustrate p^j. 3-i — 38. C, centrals; 

 R., primary radialia ; I. R., primary inter-radialia ; J.Br., inter-brachial area; I.M., infero-marginalia ; 

 8. M., supero-marginalia ; M., madreporite. 



Support seemed to be offered to this suggestion by the fact that the plates 

 could be distinguished in young Asteroidea and Ophiuroidea even Avhen they were 

 not clearly apparent in the more adult forms (Ludwig, 40). Stiirtz (79, p. 182) 

 has already shown that the evidence of Palaeontology is not in favour of the view 

 that this apical system, at any rate in the Ophiuroidea, has any real significance as 

 to ancestry. He has pointed out that no Palaeozoic Ophiuroid shows an apical 

 system, and that this appears to be a post-Palgeozoic development. My own 

 observations agree wath this. They show that the most primitive Asterozoa 

 have a leathery skin in which are embedded small irregular plates. On the 

 other hand, many primitive Asteroidea do show this apical system. It is probable 

 that the arrangement is not due to the retention of an ancestral feature, Init is 



