FISHES 179 



with the degree of difference between the habits and mode of 

 life of the male and female, and snch differences may arise 

 in many different ways. They may arise from courtship or 

 combat, or from circumstances unconnected with either of 

 these habits. 



Some fishes are viviparous, and in these, of course, fertilis- 

 ation is internal. An example of this is furnished by 

 Acanthias vulgaris, the Spiny Dog-fish, among the Elasmo- 

 branchs. Some Teleosteans are also viviparous, for example 

 the Cyprinoodonts, and the Viviparous Blenny. In all the 

 Elasmobranchs fertilisation is internal, those which are . not 

 viviparous laying large eggs similar in size and structure to 

 those of birds. Among Teleosteans, however, fertilisation is 

 usually external, with the exception of the few species which 

 are viviparous. Yet external fertilisation is not incompatible 

 with sexual dimorphism. Where, as in the case of the 

 Herring, the males and females swim promiscuously in shoals 

 discharging their reproductive elements, sexual dimorphism 

 does not exist. But in many other cases the males exhibit 

 special habits and actions related to special modes of effecting 

 external fertilisation, or protecting the eggs. In such cases 

 the eggs are more commonly rather large, heavy or adhesive, 

 as in the Lump-sucker and the Gobies. Among the numerous 

 marine species whose eggs are small and " pelagic," i.e. 

 buoyant in sea -water, special habits connected with the 

 fertilisation or protection of the eggs are not generally 

 developed, and sexual dimorphism is not common. But in 

 one such species, the Dragonet, Callionymus lyra, the sexual 

 peculiarities of the male are very marked, and they correspond 

 to very peculiar habits of sexual co-operation in the shedding 

 and fertilisation of the ova. Other cases of sexual dimorph- 

 ism associated with the production of pelagic ova are known, 

 but the habits to which they correspond have not in all been 

 yet observed. 



