106 



VERTEBRATE ZOOLOGY 



eT.o 



THE RESPIRATORY ORGANS OF FISHES 



The characteristic respiratory organs of aquatic vertebrates are 

 gills or branchiae. These structures are finely divided outgrowths of 

 the ectodermal or endodermal epithelium lining the branchial clefts. 

 The number of clefts or gill-slits varies from five to seven in number, 

 each cleft being separated from its neighbors by branchial septa. 

 The most primitive fishes have the larger number of branchial clefts 

 and the more modern types have regularly five. Heptanchus, some- 

 times mentioned as the most primitive living species of shark, has 

 seven clefts, Hexanchus, another primitive shark, has six, while the 

 elasmobranchs in general have five fully developed clefts and a vestig- 

 ial anterior first cleft called a spiracle. The spiracle or rudimentary 

 first cleft is also found among the most primitive Teleostomi (Cros- 



sopterygii and Chondrostei) , and is present 

 in the embryos of Teleostei and Holostei, 

 but is closed before hatching. In the 

 Holocephali, an aberrant group of elasmo- 

 branch fishes, the fifth cleft is closed in 

 the adult, thus reducing the number of 

 functional clefts to four. It will be re- 

 called that the pro-vertebrates have much 

 larger numbers of pharyngeal clefts, over 

 fifty pairs in Amphioxus and even larger 

 numbers in some tunicates. The cyclos- 

 tomes have on the whole larger numbers 

 of clefts than the true fishes. Though the 

 hag-fishes of the family Myxinidce have no 

 more than six pairs, those of the family 

 Bdellostomidce have as many as fourteen 

 pairs, while the lampreys all have seven 



. ... . pairs. The direction of evolution appears 



FIG. 57. External gills in * /** 



embryo torpedo, d, cloaca; to be one of reduction in number of clefts 



ex. b, external gills; el. o, elec- from around fifty in the Amphioxus-like 



trie organ; ys, yolk stalk. ances tor, fourteen to six in the cyclostomes, 

 (From Bridge, Cambridge ' . ' 



Nat. Hist., Vol. VII.) seven to five in the true fishes, and four in 



the Holocephali. 



The openings of the clefts to the exterior differ in different groups 

 of fishes. Among the elasmobranchs the usual situation is that each 



cx.b 



