418 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. Protopterus and Lepidosiren), and also of a 
Teleost, probably the Mormyrid (Hyperopisus bebe, Lacép) ;* the 
cutaneous gills of the Crossopterygii and some Dipnoi; the so- 
called external gills of such Teleosts as Cobitis, Gymnarchus 
(Fig. 239), and Heterotis, which are singularly like those of 
Elasmobranchs ; and the defensive spines which are developed 
on the seales or scutes of the trunk, and the dermal bones of 
the skull, in the young of some Plectognathi The most striking 
Fic. 238.—Newly-hatched embryo Teleost from a pelagic egg. A, Auditory organ ; 
E, eye; FM, continuous median fin; Ht, heart ; J, intestine; NV, nostril; Yh, 
yolk-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 Fishes occur in certain families 
of Elasmobranchs2 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 Gambusia 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. 180. ? See Chap. XVII. p, 434. 
3 Eigenmann, Bull. Fish Comm. (U.S.), 1892, p. 381; Arch, Entwickelungsmech. 
iv. 1896, p. 125; Cunningham, op. cit. p, 356, et seg. 
