VERTEBRATA: FISHES. 



471 



pears; and the permanent caudal increases in size, and is 

 gradually transformed from a ventral into a terminal append- 

 age, the tail -fin thus assuming its permanent " homocercal " 



Fig. 254. Tail of young Flounder (PZeuronectes) in its heterocercal stage of develop- 

 ment, a Embryonic caudal fin ; b Permanent caudal fin, occupying an inferior 

 position ; c Bent-up end of the notochord. (After A. Agassiz.) 



form. It would thus appear that the really earliest stage of 

 the tail in the Bony Fishes and Elasmobranchs is the " lepto- 

 cardial " stage, in which the tail is symmetrical and the noto- 

 chord straight. This 

 stage is in progress of 

 growth superseded by 

 the " heterocercal " 

 condition, which sub- 

 sists throughout life 

 in the Elasmobranchs. 

 Finally, the heterocer- 

 cal tail of the young 

 Bony Fish is in the 

 adult succeeded by the 



Fig. 255. Tail of adult Flounder. (After A. Agassiz.) 

 v Vertebral column ; Turned-uj end of the noto- 



chord ; h Hypural bones 



permanent "homocer- 

 cal " or " diphycercal " 

 tail. 



The process of respiration in all fishes is essentially aquatic, 

 and is carried on by means of branchial plates or tufts devel- 

 oped upon the posterior visceral arches, which are persistent, 

 and do not disappear at the close of embryonic life, as they do 

 in other Vertebrates. In the Lancelet alone, respiration is 

 effected partly by branchial filaments placed round the com- 

 mencement of the pharynx, and partly by the pharynx itself, 

 which is greatly enlarged, and has its walls perforated by a 

 series of transverse ciliated fissures. The arrangement and 



