448 



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, i. 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 bars separating the 

 clefts, are supported by the cartilaginous visceral arches 

 and branchial rays (Figs. 112, br. 

 r, and 118, 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 respira- 

 tion. As the fish swims, water 

 enters the mouth and passes by the 

 internal clefts into the branchial 

 pouches, and thence outwards by 

 the external clefts, a constant sup- 

 ply of oxygen being thus ensured . 

 The gill-pouches are developed as 

 offshoots of the pharynx, and the 

 respiratory epithelium is therefore 

 endodermal, not ectodermal,. as in 

 the crayfish, mussel and young tad" 

 pole (compare 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 incomplete 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 the anterior set of the 

 next. An entire gill or holobranch therefore consists of 

 two half-gills or hemibranchs the sets of branchial ii la- 

 ments belonging to the adjacent sides of two consecutive 

 gill-pouches (Fig. 118). On the other hand, a gill-pouch 



FIG. 118. Transverse section 

 through a gill of an Elas- 

 mobranch. 



a. afferent branchial artery ; 

 b. branchial arch ; bfl. 

 anterior, and bl z . pos- 

 terior hemibranch ; h. 

 septum ; r. branchial 

 ray ; v. efferent branchial 

 arteries. (From R. Hert- 

 wig's Zoology.) 



