RESPIRATION 37 



a feature from which the earlier name of Elasmobranchs 

 (strap gills) given to the class of Selachians was derived. 

 Reference to the accompanying diagram will show how the 

 gills are arranged in a typical Selachian (Fig. 15A), and it will 

 be observed that the anterior wall of the first pouch has its row 

 of filaments, but the posterior wall of the last pouch is not 

 provided with these structures. 



Mention may be made here of the organ known as the 

 spiracle, which is, in reality, nothing more than the vestige of 

 a gill-cleft, and, indeed, in the early embryonic stages of a 

 Shark this differs little from the clefts that lie behind it, 

 although it subsequently degenerates. Even in the adult, 

 however, the spiracle frequently retains a number of branchial 

 lamellae, and probably aids in aerating the blood going to the 

 eye and brain. It varies greatly in size in the different families, 

 being small or absent in some of the larger Sharks, and com- 

 paratively large in the Torpedoes, Rays, and Sting Rays, where 

 it has acquired a special function to be described later on 

 (Figs. 14A, b; 15A, sp.). 



The class of Marsipobranchs (Lampreys and Hag-fishes), a 

 name meaning "purse gills," exhibits a type of respiratory 

 organ which, although in some respects more primitive than 

 that of the Selachians, presents several special and peculiar 

 features. The respiratory lamellae are lodged in a series of 

 muscular pouches, well separated from each other, differing in 

 number in the various species. In the Lamprey {Petromyzon) 

 there are seven on either side, each of which opens directly to 

 the exterior by a small rounded opening on the outside of the 

 head, and communicates by a similar orifice internally, not 

 directly with the pharynx as in the Selachians, but with a 

 special canal; this ends blindly behind, but in front opens into 

 the mouth. There are, thus, seven external openings and one 

 internal opening. In the Hag-fish {Myxine) each pouch opens 

 directly into the pharynx, but on the outside it is drawn out 

 into a tubular canal running posteriorly; further back all the 

 canals unite and open together by a single external aperture 

 (Fig. 16). The gill-pouches are large in the Lamprey, and are 

 supported by an elaborate cartilaginous structure, the branchial 

 basket, which in the Hag-fish is greatly reduced. In many 

 respects this basket presents a superficial resemblance to the 

 gill-arches of the Selachians, but lies outside the gill-pouches 

 instead of between them and the pharynx. 



The general appearance of the gills in a typical Bony Fish 



