REPRODUCTION. 509 



attention to the habits and habitat of the animals, to the nature of the 

 light in which they live, and to the enemies which are likely to attack 

 them. 



Food. — The food of Fishes is very diverse — from Protozoa to 

 Cetaceans. Sharks and many others are voraciously carnivorous ; 

 many engulf worms, crustaceans, insects, molluscs, or other fishes ; 

 others browse on sea-weeds, or swallow mud for the sake of the living 

 and dead organisms which it contains. Their appetite is often 

 enormous, and cases are known (e.g. Chiasmodon niger) where a fish 

 has swallowed another larger than its own normal size. Many fishes 

 follow their food by sight ; many by a diffuse sensitiveness, to which it 

 is difficult to give a name ; a few, it would seem, by a localised sense 

 of smell. It is important to realise that fishes depend very largely on 

 small crustaceans, and these again on unicellular plants and animals. 

 Just as we may say that all flesh is grass, so we may say that all fish is 

 Diatom. 



Senses, etc. — Fishes do not seem to have much sense of taste or of 

 smell, but diffuse sensitiveness to touch, chemical stimuli, etc., is well 

 developed, especially on the head and along the lateral line. Though 

 there is no drum, and the ear is deeply buried, they certainly hear ; 

 thus there are well-known cases of tame fishes coming to the sound of 

 a bell or voice. Experiments have led some to believe that the 

 semicircular canals of the fish's ear are indispensable in the direction 

 or equilibration of movement, and it is obvious that this function is 

 more important to a fish than the luxury of listening. But the results 

 of experiment are still somewhat discordant. The sense of sight is, on 

 the whole, well developed, and many have "darkness eyes." As to 

 the intellectual powers of their small brains we know little, but many 

 show quickness in perceiving friend or foes, a few give evidence of 

 memory, and many of their instincts are complex. At the breeding 

 season there is sometimes an elaborate expression of excitement, well 

 seen in the stickleback. 



Reproduction. — Hermaphroditism occurs constantly in Ck-ryso- 

 pkrys auratus (dichogamous), and in three species of Serranus 

 (autogamous) ; almost constantly in Pagellus mormyrus ; very fre- 

 quently in Box salpa and Ckarax puntazzo ; and exceptionally in over a 

 score of fishes, such as sturgeon, cod, herring, pike, and carp. The 

 simplicity of the genital organs and their ducts may perhaps in part 

 explain why casual hermaphroditism is more frequent in Fishes than in 

 higher Vertebrates. 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 (Macropodus), 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 



