628 PHYLUM CHORDATA : CLASS PISCES — FISHES 



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 " dark- 

 ness eyes." As to the intellectual powers of their small brains we 

 know Uttle, 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 auratiis (dichogamous), and three species of 

 Serranus (autogamous) ; almost constant in Pagellus monnyrus ; 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 stickle- 

 backs {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 

 laid at once. In Elasmobranchs the egg is large, and in the oviparous 

 genera it is enclosed in a " mermaid's purse." 



Most sharks and a few Teleosteans, e.g. Sebastes marinus, Zoarces 

 viviparus, are viviparous, the eggs being hatched in the lower part of 

 the oviduct in sharks, in the ovary or oviduct in Teleosteans. In two 

 viviparous sharks {.Musielus Icevis and Carcharias glaucus) there is a 

 union between the yolk-sac and the wall of the oviduct, to be com- 

 pared with a similar occurrence in two lizards, and with the yolk-sac 

 placenta of some 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 Uttle. Moreover, the conditions of their life are not 

 suited for the development of that virtue. When it is exhibited, it is 

 usually by the males — e.g. by the sea-horse (Hippocampus) and the 

 pipe-fish {Syngnathus), which hatch the eggs in external pouches, and 

 " the male of some species of Arius, who carries the ova about with 

 him in his capacious pharynx." The female of Aspredo carries the 

 eggs on the under surface of the body until they are hatched, much in 



