OPHIUROIDEA. 



197 



apertures which in some species are double (Ophioderma) are placed on the 

 lower side of the disc, one on either side of the insertion of each arm. 

 The genital plate (Fig. 141, gp) is a skeletal piece placed on the radial side 

 of each of these slits. 



In a view from the inner side (Fig. 141) the angles are seen to consist 

 of four plates; the two oral-angle plates (am , + ad Y ) and the peristomial 

 plates (am 1 ). The oral -angle plates meet at the torus angularis (ta) and 

 diverge outwards and towards the radii. Each of them has on the side 

 towards the buccal fissure two depressions for the oral tube-feet (Fig. 141, 

 pteb) and carries on its lower edge a number of small spines ; of these some 

 project into the buccal fissures and are called oral papillae (Fig. 142, 10), 



ptel 



FIG. 141. Oral skeleton of Ophiopyren longispinus Lym., from within (from Lang after 

 Lyman). am ambulacral ossicle ; am i peristomial plates, supposed to be the ambulacral 

 ossicle of the first brachial segment ; am 2 + adi oral-angle plates ; D teeth ; fb bursal aper- 

 tures ; gp genital plate ; ibr interbrachial region ; pteb depressions for oral tube-feet ; 

 sge bursal plate ; to torus angularis. 



while others arise nearer the angle and project towards the centre of the 

 mouth and are called dental papillae. The dental papillae are above the 

 teeth of the torus angularis. The oral-angle plates are supposed to con- 

 sist of the adambulacral (lateral) plate of the first brachial segment (next 

 the angle) and of the ambulacral ossicle of the second brachial segment 

 (outer part of oral-angle plate). 



The peristomial plates (am^) are on the inner side of the oral-angle 

 plates and are supposed to represent the ambulacral ossicles of the first 

 brachial segment. 



At the outer end of each buccal fissure is a plate (Fig. 142, 8) which is 

 supposed to be the under plate of the second brachial segment) ; dorsally 

 to this there may be sometimes made out another piece which is supposed 



