THE ELASMOBRANCII FISHES 



151 



opened jioeket in HcptdiicIiHs. In tlie rays the pocket is somewhat like an in- 

 verted U, in which the internal and external branchial apertures are repre- 

 sented by the ti])s of the IT close together. 



The first indication of a pocket in the embryo appears as an evagination or 

 outpocketing of the pharyngeal wall toward the exterior (gp., fig. 144). As 

 this approaches the surface it meets a slight pitting in from the outside, and 

 the wall between the two breaks through to form the external branchial cleft 

 or aperture. 



In pentanchid types six of these pouches, including the s])iracular, are 

 formed; but accessory ]>ockets are often indicated. 

 Thus in Acanthias a small i^oucli on the left side 

 (or a pair of pouches) is produced as an evagina- 

 tion from the floor of the pharynx just back and 

 mediad of the sixth pouch. This pouch does not 

 reach the outside layer but comes in contact with 

 the roof of the pericardial cavity. Here it forms 

 numerous tubules and becomes the so-called post- 

 branchial or suprapericardial body. These bodies 

 have also been described for Scyllium, Galeus, 

 Pristiurus, and Raja. 



In figure 144 the anterior clefts have thus broken 

 through. Between the pockets are columns, from 

 which all the tissues of the holobranchs are later 

 produced including their supporting cartilages 

 and musculature. On the posterior wall of the 

 hyoidean cleft will appear a little later the begin- 

 nings of the embryonic or external gill filaments. 



The whole column from the internal branchial 

 arch toward the exterior lengthens out and the 

 central core becomes a plate, the septum or dia- 

 phragm of the gill. This plate supports the fila- 

 ments or respiratory membrane, and is peculiar in 

 the Elasmobranchs in that it extends outward 

 beyond the filaments which are attached to it. It is 

 from the peculiar attachment of the filaments to the septa that the Elasmo- 

 branchs have received their name ('EXacrjuos: blade- or strap-like; ^payxt-o-: 

 gilled). The septum may be said to attach internally on the internal visceral 

 arches, and to extend outward to the lower layer of the integument. Each 

 septum is limited anteriorly and posteriorly by a heavy layer of tissue which 

 is continuous with the mucous membrane of the pharynx. Terminally the an- 

 terior and posterior plates of this tissue run parallel and are separated only 

 by the interbranchial musculature, connective tissue, and cartilaginous bran- 

 chial rays {Heptanchus) which support the septum. 



The interbranchial muscles (ib.m., fig. 145) are located on the anterior sur- 



Fig. 144. Horizontal section 

 through pharynx to show for- 

 mation of the gill pockets, 

 Hcterodontus francisci. (Mar- 

 shall Williamson, del.) 



h.v., blood vessels; c, cav- 

 ity; gp., gill pocket. 



