THE LIFE OF FISHES. 413 



The sexes are separate, except in Chrysophrys and 

 Serranus, two hermaphrodite bony fishes, or when ab- 

 normal hermaphroditism occurs, as in herring and mackerel. 

 In many cases the males are smaller, brighter, and less 

 numerous than the females. Courtship is illustrated by the 

 sticklebacks {Gasterosteus, etc.), the paradise-fish {Macro- 

 podus), and others, while the bent lower jaw of the male 

 salmon reminds us that some male fishes fight with their 

 rivals. 



Most Fishes lay eggs which are fertilised and develop 

 outside of the body. They may be extruded on gravelly 

 ground or sown broadcast in the water. Sturgeon, salmon, 

 and some others ascend rivers for spawning purposes, while 

 the eels descend to the sea. In the case of trout, Barfurth 

 has observed that the absence of suitable spawning ground 

 may cause the fish to retain its ova. This results in ovarian 

 disease and in an inferior brood next season, a fact which 

 should be compared with what Hertwig has observed in 

 regard to Echinoderms, that ova which are retained beyond 

 the normal period become over-ripe and pathological. 

 Except in Elasmobranchs the ova are relatively small, and 

 large numbers are usually laid at once. In Elasmobranchs, 

 the egg is large, and in the oviparous genera it is enclosed 

 in a "horny " sheath, familiarly illustrated by the mermaid's 

 purse of the skate. 



Most sharks and a few Teleosteans are viviparous, the 

 eggs being hatched within the body of the mother, — in 

 the lower part of the oviduct in sharks, in the ovary in 

 Teleosteans. In two of the viviparous sharks (Mustelus 

 Icevis and Carcharias glaucus), there is an interesting union 

 between the yolk-sac and the wall of the oviduct, which 

 should be compared with a similar occurrence in two lizards, 

 and with the placenta of most Mammals. 



As to fertilisation, the usual process is that the male 

 deposits spermatozoa or "milt" upon the laid eggs or 

 " spawn," but fertilisation is of course internal when the eggs 

 are enveloped in a firm sheath, or when they are hatched 

 within the mother. 



Most Fishes have a great number of offspring, and parental 

 care is proportionately little. Moreover, the conditions of 

 their life are not suited for the development of that virtue. 



