THE ELASMOBRANCH FISHES 



221 



gills and gill-pouches can be studied in figs. 53, 54, and 55, 

 but their exact relations can only be understood after careful 

 dissection. Each gill-pouch or branchial cleft is formed as 

 an outgrowth of the throat, which meets and fuses with the 

 epidermis, and eventually opens to the exterior by an aperture 

 formed in the line of fusion. The pouches, therefore, are 



els 



Fig. 53 



A. A single gill partition from the left side of Scy Ilium canicula., seen from in 

 front, bra, branchial arch ; ebi, inferior extra-branchial cartilage ; ebs, 

 superior extra-branchial cartilage ; gl, vascular folds forming the gill or 

 demi-branch. />'!, E& ', Z? 3 . Three views of a denticle from the skin. />, 

 basal plate ; dt, dentinal tubes ; PC, pulp cavity. 



lined by a hypoblastic epithelium or mucous membrane con- 

 tinuous with that of the pharynx. The transition from this 

 membrane to the epidermis can easily be seen when the gills 

 are slit up, the epidermis being covered with denticles, while 

 the mucous membrane of the gill-pouches is smooth and 

 moist. The partition wall between any two contiguous gill- 

 pouches is thick on the internal or pharyngeal side, owing to 

 the presence of the cartilaginous visceral arch, but the re- 



