GENERAL NOTES ON FISHES. 561 



distribution of the pigment granules in the cells is altered. In shallow 

 and clear water this power of colour-change may be protective, but an 

 appreciation of the protective value of colouring demands careful 

 attention to the habits and habitat of the fishes, 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 engult worms, crustaceans, insects, molluscs, or other fishes ; 

 others browse on seaweeds, 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, a few seem to hear. 

 Some experiments suggest that the semicircular canals of the fish's ear 

 are indispensable in the direction or equilibration of movement. 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 friends 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 is constant in some bony 

 fishes, e.g. Chrysophrys auratus (dichogamous), and three species 

 of Serranus (autogamous) ; almost constant in Pagellus mormyrus ; 

 very frequent in Box salpa and Charax puntazzo ; and exceptional 

 in over a score of fishes, such as sturgeon, cod, herring, pike, and 

 carp. The simplicity of the genital organs and their ducts may 

 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 ; and many 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. Except in Elasmo- 

 branchs, the ova are relatively small, and large numbers are usually 



