432 



THE DOGFISH 



Kespiratory Organs. The gills consist of five pairs of 

 pouches, each opening by one of the internal branchial 

 apertures (Fig. io8, i. br. a) into the pharynx, and by one 

 of the external branchial apertures on the exterior. The 

 walls of the pouches, or iitter-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 artery; 

 b. branchial arch ; bt^. 

 anterior, and b?^. pos- 

 terior hemibranch ; h, 

 septum ; r. branchial 

 ray ; v. efferent branchial 

 artery. (From R. Hert- 

 wig's Zoology.^ 



As already mentioned, the walls of the pharynx 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-pouches, each being related to the posterior set of filaments of one 

 pouch and theanterior set of the next. An entire gill or Ao/i5^ra«i:,4 therefore 

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

 belonging to the adjacent sides of two consecutive 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. 



