IX 



TEETH, GILL-GLEFTS 



299 



thickness of pharyngeal and body wall, there being no intervening 

 coelomic space in the adult in this region. The external openings, which 

 become somewhat restricted in dorsiventral extent as development pro- 

 ceeds, are seen arranged one behind 

 the other along the side of the body 

 just in front of and slightly dorsal 

 to the pectoral fin (Fig. 121, v.c). 

 The five clefts are separated from 

 ^•'■■■-.M \ one another by four gill-septa. 



wpJL 



Each gill septum is thickest along 

 its inner or pharyngeal edge and 

 this edge carries in front and behind 

 a row of conical projections termed 

 gill-rakers (Fig. 124, B, g.r) which 

 project across the pharyngeal opening 

 of the cleft and serve to prevent 

 blocks of food material from passing 

 outwards into the cavity of the cleft. 

 In Acanihias both the anterior and 

 the posterior row of gill-rakers are 

 well developed : in Scyllium those 

 of the posterior row are incon- 

 spicuous. 



The actual respiratory surface through which gaseous interchange 

 takes place between the blood and the water is afforded by the flat face 

 of the septum on each side. In accordance with this the specially 

 respiratory portions of this surface are provided with a rich network of 



Fig. 124. 



A, Horizontal section through gill-clefts ot 

 Acanihias. B, portion of similar section show- 

 ing gill-clefts of left side on a larger scale, 

 g, Gill ray supporting the septum; g.r, gill- 

 rakers ; M, mouth ; w, muscles of branchial arch ; 

 oes, oesophagus ; olf, olfactory organ ; p.g, pec- 

 toral girdle ; ph, pharynx ; r.l, respiratory 

 lamella ; v.a, cartilage of visceral arch ; v.c, vis- 

 ceral cleft. 



