XVII OPHIOTHRIX DISC ALIMENTARY CANAL ' 485 



termed " scuta buecalia " (Fig. HI 1, </^ ), on one of which open tlie 

 few niadreporic pores wliich tlie animal possesses. Attaclied to 

 the sides of the scuta buecalia are the " lateral mouth shields," 

 which are in fact the adambulacral plates belonging to the second 

 pair of ambulacral plates which form the main mass of the jaws. 

 Further out, on the under side of the disc, there is, on each side 

 of each arm, a long narrow slit — tlie opening of the genital bursa 

 (Fig. 208, (jl)), so that there are ten genital bursae. The 

 "genital bursa" (Fig. 214) is a sac lined by ciliated ectoderm 

 projecting into the interior of the disc. It is called genital 

 because the openings of the genital organs are situated on its 

 surface ; its main function, however, is respiratory, the cilia 

 bringing about a constant inward current of fresh sea-water, 

 the oxygen contaiiied in which diffuses through the thin wall 

 of the sac into the coelomic fluid. The opening of the bursa is 

 strengthened on its radial side by a rod-like ossicle, the " genital 

 plate," and on its interradial side by an ossicle called the 

 "genital scale" (Fig. 212, ge%), and in Oi^hiothrix the outer end 

 <jf the radial plate articulates with the outer end of the genital 

 plate. Muscles connect the two plates running on either side 

 of the articulation. 



Observations on Oj^hiothrix^ show that in this species at any 

 rate the radial plates can be raised or lowered. Wlien they are 

 raised the centre of the disc is lifted into a cone and water is 

 sucked into the genital bursae, whereas when they are lowered 

 the bursae are compressed and water is expelled. This forced 

 respiration appears to come into play when tlie supply of oxygen 

 is getting scanty. 



The alimentary canal of Opliiotlirix is a simple flattened 

 sac (Fig. 213). It is devoid of an anus and cannot be everted 

 through the mouth. There is a horizontal pouch given off into 

 each interradial lobe of the disc. The sac is attached to the 

 dorsal wall of the coeloni by numerous mesenteries, fibrous cords 

 traversing tlie coelomic cavity and clothed on the outer side by 

 coelomic epithelium. To the mouth-frame it is attached by a 

 circular membrane, whicli we have reason for believing is a 



^ How far this form of respiratory mecliaiiisni is distributed amongst Opliiurids 

 it is imiJossible to say. It was first observed by me in tlie case of Ophiothrix 

 frayUis at Plymouth in 1905, but since then I have found it in Ophiuru ciliaris 

 and in Amphiura squancaia. 



