CHAPTER V 



MODES OF REPRODUCTION 



Oviparous and viviparous Elasmobranchs ; eggs of Chimaeroids. Nests of 

 Protopterus and Lepidosiren. Nest-building of Ganoids and primitive fresh- 

 water Teleosteans. Parental instincts in British shore-fishes. Nest of the Sar- 

 gasso. Fishes which incubate the eggs in the mouth. Pouch of the Pipe-fishes. 

 Viviparous fishes. Curious breeding habits of the Bitterling. 



THE sharks, dog-fishes and skates (Elasmobranchii) and 

 the Chimaeroids (Holocephali) possess oviducts which 

 are distinct and separate from the ovaries. Fertilisa- 

 tion is internal, the milt being introduced into the oviducts by 

 means of the copulatory organs which are posterior outgrowths 

 of the pelvic fins. The eggs as they leave the ovaries, are usu- 

 ally of considerable size, and resemble the yolk of a bird's egg. 

 In some species the fish are oviparous, and in these each ovi- 

 duct contains a special gland which secretes a shell of horny 

 texture and of flattened oblong shape, in which the egg, together 

 with some albuminous secretion, is enclosed ; the egg when laid 

 has thus a structure quite similar to that of a bird's egg, but the 

 shell is of different shape and contains no lime ; it is therefore 

 tough, but not brittle. In the common ground dog-fishes of 

 the genus Scyllium the corners of the egg-shell are prolonged 

 into long slender tendrils which are coiled round fixed objects or 

 seaweeds at the bottom of the water, and thus anchor the egg 

 during development. The development occupies several 

 months and the young fish is hatched in a condition resembling 

 that of the parent. In other cases the female is viviparous, the 

 development of the egg taking place within the oviduct, and in 

 some of these species, as for instance in the common spiny 

 dog-fish, AcantJiias vulgaris, a rudimentary egg-shell is formed 

 around the egg in its early condition ; the young when born 

 are fully developed and able to seek their own food ; seven or 

 eight young are produced at a birth, each of them several inches 



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