. The Cambridge natural history. Zoology. ELASMOBRANCHII 433 a single egg, but in Rhinohatus As a rule each egg-case has but and Trygonorliina (Batoidei), both of which are viviparous, each case contains three to four eggs. Generally the egg-cases are somewhat quadrangular in shape, with the four angles, two at each end, prolonged either into short horns, or into long tapering tendrils (Fig. 246). The oval egg-cases of the Heterodontidae are remarkable not only for their size, but also for the presence of a broad spiral lamina winding round the exterior of the case from one pole to the other (
. The Cambridge natural history. Zoology. ELASMOBRANCHII 433 a single egg, but in Rhinohatus As a rule each egg-case has but and Trygonorliina (Batoidei), both of which are viviparous, each case contains three to four eggs. Generally the egg-cases are somewhat quadrangular in shape, with the four angles, two at each end, prolonged either into short horns, or into long tapering tendrils (Fig. 246). The oval egg-cases of the Heterodontidae are remarkable not only for their size, but also for the presence of a broad spiral lamina winding round the exterior of the case from one pole to the other (Fig. 245). The majority of the Sharks, Dog-Fishes, and Eays are viviparous, that is, the young are born alive ; the rest, like the Scylliidae ( th^ common British Dog-Fishes, Scyllium canicula and S. catulns), the Heterodontidae, and the Eaiidae are oviparous, that is, the young are hatched out after the extrusion of the eggs. In the oviparous species the eggs are extruded either singly or in pairs, and generally deposited on the sea-bottom. When, however, the egg-cases are provided with tendrils, as, for example, in the two British Dog-Fishes just men- tioned, these organs act as anchor- ing filaments. When extruding an egg, the female swims round and round some piece of upright seaweed, and the curling tendrils become entwined round it in such a way that the egg becomes securely attached thereto (Fig. 246).^ The embryos are long in developing, and in Scyllium it may be' several months after fertilisation (200 to 275 days) before they are hatched, the young Fish finally escaping through a rupture in the egg-case. In the oviparous species the nutritive food-yolk stored up, first in the egg and subsequently in the yolk-sac (Fig. 248), suffices for the nourishment of the embryo until the period of hatching, 1 Cunningham, Marketable Marine Fishes, London, 1896, p. 64. VOL. VII 2 F. Fig. 245. — Egg-case of Ileterodontus (Cestracion) galeatus. (From Parker and Haswell, after W
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
