150 THE ELASMOBRANCH FISHES 



RESPIRATORY TRACT OF ELASMOBRAXCHS IN GENERAL 



The respiratory tract or passageway through which the water current passes 

 in bathing the gills of Elasmobranchs, unlike the tract in higher forms in 

 which the respiratory current may enter external nares, begins with the 

 mouth and terminates with the external branchial clefts. The following parts 

 are included in the respiratory tract : the buccal cavity, the enlarged pharynx, 

 and, in the wall of the pharynx, the gill pockets. The gill pockets, as in Heptan- 

 chus, are reached from the pharynx by the internal branchial apertures and 

 open to the exterior through the external branchial apertures or clefts. We 

 shall describe first the external branchial apertures. 



Many of the Elasmobranchs are of the pentanchid type, that is, those 

 possessing five gill clefts; only Chlamijcloselachus and those sharks belonging 

 to the notidanids exceeding this number. Record is also made of a greater 

 number occasionally occurring in other forms. In Chlamydoselachus and in 

 Hexanchus six clefts are present and in Heptanchus, as we have seen, there 

 are seven, the greatest number known for any gnathostome or jaw-possessing 

 vertebrate. 



The position of the external branchial apertures or clefts has long been used 

 in separating the sharks from the rays. Characteristic of the former the clefts 

 are lateral in position, while in the latter they are ventrally located. In the 

 raj'S, however, there enters the element of time, for while a ventral position of 

 the clefts is characteristic of the adult, the position for a considerable period 

 of time in the embryo is lateral (see p. 11) . 



The external branchial apertures in the sharks vary greatly in relative size. 

 In a type like Acanthias (fig. 5) they extend as slits practically one-half the 

 height of the bod3\ In Heterodontus (fig. 17) the first cleft is large, but the 

 last is so small as to be of slight functional value. Many other forms are like 

 Heterodontus in this regard. In some of the other sharks the clefts are of 

 immense size. This condition is found especially in the lamnoids and in Ceto- 

 rhinus and Rhinodon (fig. 3), in all of which the apertures are practically 

 the height of the pharj-ngeal diameter. In the rays the clefts are relatively of 

 a much smaller size. 



The first cleft to open and one of the most important in the emliryo is the 

 spiracle. As growth proceeds, however, the spiracle fails to keep pace ^^^.th the 

 other clefts, so that at its maximum development in types like Acanthias (see 

 p. 11, fig. 22) and Squatina it is relatively small. In others, as for example 

 Lamna and Carcharias, the spiracle of the adult is often minute; and in still 

 others all superficial trace of it may be lost. In the rays, on the contrary, it 

 has assumed a secondary function and hence has l^ecome enlarged. 



Gill Pouch or Pocket 



In shape the gill pockets of the sharks are generally like those of Heptanelius. 

 In types like Lamna, Cetorhinus, or Rhinodon, however, the external clefts 

 are so large that the normal form of the closed pocket is more like that of the 



