BREEDING 413 



vantages attendant on the sexual relations of Fishes, involving a 

 considerable waste of the sex-cells, while in many Fishes it no 

 doubt helps to compensate for any subsequent mortality among 

 the larvae, which may result from an uncertain and precarious 

 food supply and from the attacks of enemies. Whenever internal 

 fertilisation is the rule, or when, as in nest-building and mar- 

 supial Fishes, the propinquity of the sexes in the breeding season 

 ensures the fertilisation of a larger proportion of the eggs and the 

 protection of the young, the number of eggs produced is small. 



The male sex-cells or spermatozoa are essentially similar to 

 those of other Vertebrates, although in different Fishes they may 

 vary in such details as length, and the shape and size of the 

 head, which may be rod-like and wavy, elliptical or globular. 



As a rule, in Fishes females are more numerous than males, 

 and generally they are larger, but to both statements there are 

 notable exceptions. The relations of the sexes in the breeding 

 season are usually very promiscuous, especially in those Teleosts 

 which discharge their sex -cells while swimming together in 

 shoals. A female may, however, consort with several males 

 (polyandry}, or a male with several females (polygamy} ; or, as in 

 some of the nest-building Fishes (e.g. Gastrosteus), there are not 

 wanting examples of the pairing of one male with one female 

 (monogamy}. 



Fishes often migrate at the commencement of the breeding- 

 season to localities most suitable for the deposition of the eggs. 

 Many marine species seek banks or shallower water near 

 the shore, and some, like the Salmon and the Sturgeon, are 

 anadromous, and ascend rivers for long distances to deposit their 

 spawn. 



In all Fishes except the Elasmobranchs and a few Teleosts 

 the fertilisation of the eggs takes place in the water after their 

 extrusion, the male depositing its seminal fluid over the eggs or 

 in their neighbourhood. The waste of the sex -cells is often, no 

 doubt, very considerable, especially when the eggs are adhesive 

 and fixed, and the seminal fluid is liable to drift at the mercy of 

 tides and currents. With pelagic ova the waste is perhaps not 

 so great, inasmuch as the eggs as well as the spermatozoa 

 would probably drift at the same rate and in the same direction. 

 Liability to waste must also be greatly diminished in many 

 Fishes by their habit of living in shoals, or of congregating 



