432 



THE DOGFISH 



Respiratory Organs. The gills consist of five pairs of 

 pouches, each opening by one of the internal branchial 

 apertures (Fig. io8, /. lir. a) into the pharynx, and by one 

 of the external branchial apertures on the exterior. The 

 walls of the pouches, or inter-branchial septa, are supported 

 by the visceral arches and bran- 

 chial rays (Fig. 103, br. r and 

 109, r), and are lined with mucous 

 membrane raised into horizontal 

 ridges, the branchial filaments, 

 which are abundantly supplied with 

 blood-vessels and are the actual 

 organs of respiration. As the fish 

 swims, water enters the mouth and 

 passes by the internal clefts into the 

 branchial pouches, and thence out- 

 wards by the external clefts, a constant 

 supply of oxygen being thus ensured. 

 The gill-pouches are developed as 

 offshoots of the pharynx, and 

 the respiratory epithelium is there- 

 fore endodermal, not ectodermal, 

 as in the crayfish and mussel (com- 

 pare also pp. 204 and 207). 



Fig. 109. — Transverse section 

 through a gill of an Ela- 

 smobranch. 

 a. afferent branchial .irtery; 

 b. bianchial arch ; hl^. 

 anterior, and bt^. pos- 

 terior hemibranch ; li. 

 septum : r. branchial 

 ray ; z>. efferent branchial 

 artery. (From R. Hert- 

 vvig'.s Zoology^ 



As already mentioned, the walls of the pharyn.\ are supported by the 

 cartilaginous visceral arches, which surround it like a series of incom- 

 plete hoops, each half-arch being embedded in the inner or pharyngeal side 

 of an interbranchial septum. Thus the visceral arches alternate with the 

 gill-pouche.s, each being related to the posterior set of filaments of one 

 pouch and the anterior set of the ne.\t. hnitn^\it%i\\oxholobraiuh therefore 

 consists of two half-gills or liaiiibraiichs — the sets of branchial filaments 

 belonging to the adjacent sides of two consecuti\e gill-pouches (Fig. 109). 

 On the other hand, a gill-pouch is equivalent to the posterior hemibranch 

 of one gill and the anterior hemibranch of its immediate successor. 



