CHAPTER IV 



CONDITIONS OF LIFE AND PHENOMENA OF SEX 



Food and feeding. Relations to temperature, light and salinity. Influences 

 of seasons ; migrations, hibernation and aestivation. Association and com- 

 mensalism, parasitism. Relations of sexes, sexual dimorphism. 



AS so many fishes feed on fishes it seems at first difficult 

 to understand how their vast hosts are maintained and 

 why they have not long ago become extinct in con- 

 sequence of mutual slaughter. Ultimately all animal life de- 

 pends on the material and energy supplied by plants, which 

 alone are able by the aid of the rays of the sun to produce 

 organic substances from inorganic materials. Few fishes are 

 however directly herbivorous ; the Australian lung-fish Cera- 

 todus devours water-weeds, and it is usually stated that it does 

 this for the sake of the minute animals living among them, but 

 no reason is given for the conclusion that it does not digest 

 the weeds themselves; in the Zoological Gardens of London, 

 Cevatodus eats lettuce as a regular part of its diet. Grey 

 mullets certainly feed partly on the small green algae which 

 coat the surfaces of stones and wooden piles in harbours and 

 estuaries and also on mud, which they swallow for the organic 

 matter, largely consisting of diatoms, which it contains ; these 

 fishes have feeble teeth or none, and the stomach forms a strong 

 gizzard like that of a bird. The Cyprinidae or carps have no 

 teeth in the jaws at all, and the teeth in the pharynx vary with 

 the nature of the food ; many of them feed on mud and vegetable 

 substances ; in these the pharyngeal teeth are adapted for masti- 

 cation and the intestines are long and convoluted. Dorosoma 

 cepedianum, belonging to the Clupeidae and living in the Mis- 

 sissippi, is commonly called the hickory-shad because the 

 stomach forms a gizzard of the size and shape of a hickory 

 nut ; it and its allies feed on mud, they have a small contracted 

 toothless mouth, and their nourishment must depend largely on 



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