41 8 FISHES CHAP. 



provided with larval or provisional organs, and they may be so 

 unlike the adult in other respects that their subsequent develop- 

 ment assumes the form of a more or less striking metamorphosis. 

 As examples of larval organs, mention may be made of the adhesive 

 or cement organs of the larval Chondrostei and Holostei, and of 

 the Dipnoi (e.g. Protofterus and Lepidosiren), and also of a 

 Teleost, probably the Mormyrid (Hyperojnsus hebe, Lacep) ; -^ the 

 cutaneous gills of the Crossopterygii and some Dipnoi ; the so- 

 called external gills of such Teleosts as Cobitis, Gymnarchus 

 (Pig. 239), and Heterotis, which are singularly like those of 

 Elasmobranchs ; and the defensive spines which are developed 

 on the scales or scutes of the trunk, and the dermal bones of 

 the skull, in the young of some Plectognathi. The most striking 



Fig. 238. — Newly-hatched embryo Teleost from a pelagic egg. A, Auditory organ; 

 E, eye ; FM, continuous median fin ; Ht, heart ; 1, intestine ; N, nostril ; Tk, 

 yollt-sac. (From Cunningham.) 



metamorphosis to be found in Fishes occurs in the Flat-Fishes 

 and in the Eels, an account of which will be found in other 

 parts of this volume (pp. 685, 602). 



The only examples of viviparous Pishes occur in certain families 

 of Elasmobranchs,^ and in five families of Teleosts, viz. the Blen- 

 niidae, the Cyprinodontidae, the Scorpaenidae, the Comephoridae, 

 and the Embiotocidae.^ In the Teleosts mentioned the eggs are 

 fertilised while they are still either in the ovarian ovisacs or in 

 the cavity of the ovary, and their development may take place in 

 either position. In such Cyprinodonts as Gamhusia and Anableps 

 the embryos are developed in the ovisacs, but as a rule both 

 fertilisation and development occur in the ovarian cavity. During 

 a prolonged gestation the young are nourished partly by the food- 



1 Budgett, Trans. Zool. Soc. xvi. Pt. ii. 1901, p. 130. = See Chap. XVII. p. 434. 

 " Eigenmann, Bnll. Fish Coinm. (U.S.), 1892, p. 381 ; Arch. Entwichelungsmech. 

 iv. 1896, p. 125 ; Cunningham, op. cit. p. 356, et seq. 



