THE DOGFISH 



CHAP. 



Respiratory organs. The respiratory organs consist of 

 five pairs of pouches, each opening by one of the internal 

 branchial apertures (Fig. 117, /. br. a) into the pharynx, and 

 by one of the external branchial apertures on the exterior. 

 The walls of the pouches, or inter-branchial septa, which 

 form the arches separating the clefts, are supported by the 

 cartilaginous visceral arches and 

 branchial rays (Figs. 112, br. r, and 

 1 1 8, 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). 



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 imbedded 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 the anterior set of the next. A n entire gill or ho lo branch therefore 

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

 belonging to the adjacent sides of two consecutive gill-pouches (Fig. 118). 

 On the other hand, a gill-pouch encloses the posterior hemibranch of one 

 gill and the anterior hemibranch of its immediate successor. 



FIG. 1 1 8. Transverse section 

 through a gill of an Elas- 

 mobranch. 



a. afferent branchial artery; 

 b. branchial arch ; 3/ 1 . 

 anterior, and bft. pos- 

 terior hemibranch ; h. 

 septum ; r. branchial 

 ray ; v. efferent branchial 

 arteries. (From R. Hert- 

 wig's Zoology.) 



