198 



PHYLUM ECHINODERMATA. 



to represent the under plate of the first brachial segment. The mouth is a 

 star-shaped aperture with the five interradial angles projecting into it 

 (Fig. 142). These angles are formed externally of four plates : the oral 

 plate or buccal shield (7), of two lateral buccal plates (5) lying on the sides 

 of the buccal shield and meeting on the oral side of it, and of the torus 

 angularis, which carries teeth. The lateral buccal shields are supposed 

 to be the modified adambulacrals (laterals) of the second brachial seg- 

 ment. The torus angularis consist of a vertical row of pieces which 

 may fuse together (Fig. 141, ta). The slits between the angles are called 

 the buccal fissures. 



Fm. 142. Lower surface of disc and base of arms of Ophiactis poa Lym. seen from the out- 

 side (from Lang after Lyman). 1 under plates ; 2 spines of the lateral plates 4; 3 ten- 

 tacle scales ; 5 lateral buccal shields ; 6 apertures of bursae ; 7 buccal shields ; 8 first 

 under plate of arm (supposed to belong to tne 2nd brarhial segment) ; 9 torus angularis ; 

 10 oral papillae. 



The skeleton of the disc. The upper skeletal plates of the 

 disc consist principally of the plates of the primary apical 

 system, but there are generally other plates as well, and these 

 may be so numerous as to completely obscure the primary plates. 

 Moreover the completeness of the system of primary apical 

 plates varies even in the same genus. In a typical case the ar- 

 rangement of plates on the upper side of the disc is as follows 

 (Fig. 143) : a central plate surrounded by five radials (r) and five 

 basals (ba) ; the radials are separated from the central by the 



