. The Cambridge natural history. Zoology. 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 ( 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
. The Cambridge natural history. Zoology. 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 ( 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. (
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