152 THE ELASMOBRANCH FISHES 



face of, and run at right angles to, the cartilaginous branchial rays of each 

 whole gill. These muscles form thin sheaths dorsally and ventrally by which 

 attachment is made to the fascia and to the extrabranchial cartilages; medially 

 a part of the interbranchial joins the epi- and ceratobranchial segments of the 

 internal branchial arches. Some of the outer fibers in their course are attached 

 to cartilaginous branchial rays, but in the sharks most of them are continuous 

 above and below. In the rays these muscles are regularly attached to the bran- 

 chial rays (see p. 106, fig. 108b) . They are thus able by contraction to decrease 

 the size of the pocket. 



The filaments of the adult gill are to be distinguished from those of the early 

 embryo which we may first consider. Upon the breaking through of the ex- 

 ternal clefts in the embryo of all Elasmobranchs a series of nodules arises from 

 the posterior margins of the hyoidean and the branchial arches. These grow 

 outward as long external gill filaments which sometimes exceed the entire 

 body in length {JJrolophus, fig. 22d) ; in others they are shorter [Acanthias, 

 fig. 22c). Such filaments are characteristically more numerous in the more 

 generalized types. In Heptanch xs we have seen the greatest number known in 

 Elasmobranchs. In Acanthias they are more numerous than in Scylliuni, and 

 in Scyllium they exceed in number those of most rays. 



These filaments in the living embryo are particularly noticeable from the 

 fact that they are kept constantly in motion and are filled with blood, giving to 

 them their striking color. Under the microscope the circulation of blood can 

 here be seen to remarkable advantage. Each embryonic filament is a more or 

 less flattened plate with a blood vessel encircling it near its outer border. The 

 whole tissue of the filament acts as the respiratory membrane. 



External branchial filaments thus developed in the embryo are early ab- 

 sorbed, giving place to permanent or internal filaments. In some forms the 

 filaments of the adult may be of great length reaching practically to the outer 

 margin of the septum (Lamna). In many others, however, they are of medium 

 length (Scyllium); while in a few they are relatively short (Sguatma). 

 Usually in the Elasmobranchs the filaments of the posterior demibranch are 

 longer than those of the anterior; and those of the middle of the septum are 

 the longest of the series. 



Sections through the septum parallel to the filaments give a clear notion of 

 the structure of a gill. Droscher (1882) gave such a section through a holo- 

 brancli of Torpedo (fig. 145) which shows that here the filament of the pos- 

 terior demibranch, like that in Heptanchus, is longer than that of the anterior, 

 and that both extend only two-thirds the length of the septum. Running 

 transversely through the central part of the septum, from the internal bran- 

 chial arch toward the exterior is the interbranchial muscle; back of this is the 

 cartilaginous branchial ray. It is observed further that the filaments, instead 

 of being round as in face view, are flattened from side to side. A section of the 

 filaments of Torpedo, so far as considered, is essentially like the one studied 

 of Heptanchus. It differs from it somewliat in the position of its blood and in 

 its nerve supply. 



