148 THE ELASMOBRANCH FISHES 



is located dorsally and extends inward until it meets and joins the cartilage of 

 the auditory capsule. A second smaller diverticulum also extends dorsally 

 from the spiracular walls near their union with the pharynx. 



Gill or Holobranch 



A holobranch consists of the tissue between two gill pockets; for example, the 

 tissue between the second and third pockets constitutes the second gill or holo- 

 branch. This second holobranch hence includes all filaments on the posterior 

 wall of the second pocket, and those on the anterior wall of the third gill 

 pocket, as well as the supporting structures between the two. The filaments on 

 the anterior and posterior sides of a whole gill are also designated as the an- 

 terior and posterior demibranchs. Thus the filaments on the posterior side of 

 the second pocket form the anterior demibranch of the second gill, and the fila- 

 ments on the anterior side of the third pocket constitute the posterior demi- 

 branch of the second gill. 



With this understanding of a gill there are thus present in Heptanchus six 

 whole gills, located between pockets 1-2, 2-3, 3-4, 4-5, 5-6, 6-7. In addition 

 there is present on the anterior wall of the first pocket a half -gill, the hyoidean 

 demibranch. It is often more convenient to consider the number of demi- 

 branchs rather than the number of holobranchs. Thus there are present in ad- 

 dition to the first unpaired hyoidean demibranch, a second and third, a fourth 

 and fifth, a sixth and seventh, an eighth and ninth, a tenth and eleventh, and a 

 twelfth and thirteenth branchial demibranch, these pairs representing the 

 first, second, third, fourth, fifth, and sixth gills or holobranchs, respectively. 



The gill filaments of the adult differ from certain external filaments present 

 in the embryo. The external filaments of the emliryo arise from the tissue on 

 the posterior part of the hyoidean and branchial arches, and in Heptanchus 

 are of particular interest because of their great numbers, a mark of a general- 

 ized condition. Those filaments arising on the hyoid in Heptanchus cinereus, 

 according to Braus (1906), consist of 14 filaments; those from the first bran- 

 chial opening, 39; from the second to the fifth, 29 each; from the sixth, 26, and 

 from the last, 18. 



Gill Supports 



The arches supporting the gills of the adult (see fig. 48, facing p. 44, and 

 fig. 49, p. 45) are the hyoidean and all the branchial arches except the last. 

 From epi- and ceratobranchial segments of all the visceral arches, except the 

 mandibular and the last branchial, cartilaginous branchial rays (b.r.) extend 

 outward toward the integument. The branchial rays on the hyoidean arch 

 (fig. 48) are often branched and complex, but on the other visceral (branchial) 

 arches they are simple unbranched rods. In the latter they are separated from 

 the gill filaments anteriorly by the interbranchial musculature and extend as 

 supports beyond the ends of the filaments. 



The septum of each holobranch, or wall between two sets of filaments, ex- 



