Chap. VI.] 



GILLS OF FISHES. 



233 



the gill sacs open separately to the exterior, but in 



the hag the ducts unite and open by a common orifice 



on either side. A similar modification obtains in the 



gnathostomatous fishes, for in the 



shark -like elasmobranchs (sharks and 



rays) each gill cleft is open to the 



exterior, while in Chimsera and all 



other Fishes the clefts are covered 



over either by a fold of the skin 



merely, or by bony pieces (opercu- 



liini), so that there is on either side 



but a single opening to the exterior. 



The gills, when complete, consist 

 of two folds of mucous membrane, 

 which are ordinarily triangular in 

 shape (Fig. 98), are supported by a 

 branchial arch (b), and by cartila- 

 ginous rods, have a blood-vessel 

 passing into them and bringing blood 

 from the heart, which breaks up 

 into capillaries ; these capillaries 

 unite into another vessel which car- 

 ries the oxygenated blood back to 

 the body. A gill is said to be 

 incomplete when one of the two 

 folds is alone developed. 



IntheCyclostomata and the shark 

 like Elasmobranchs, the gills take the 

 form of pouches, and the lamellae 

 form the transverse folds on the 

 walls of the pouch ; the septa which 

 project from the branchial arches are 

 as long as the gills, and each gill 

 chamber is, therefore, in a shark, completely separated 

 from the one in front or behind it ; as we pass through 

 Chimaera and the Ganoids to the Teleostei, we find 

 that these septa become more and more reduced, so that 



the form of the 

 gills and the ar- 

 rangement of the 

 blood-vessels. 



a, Branchial artery ; 

 6, branchial arch 

 (seen in cross sec- 

 tion); c, branches of 

 the branchial vein 

 v ; d, branches of 

 branchial artery. 



