S UR VE Y OF FISHES. 5 1 5 



the bottom ? Or was the change due to certain peculiarities of structure, 

 — requiring, of course, previous explanation, — such as the great depth of 

 the body and the degeneration of the swim-bladder? Or did both 

 these causes operate at once ? 



But supposing we had attained to some clearness in regard to the 

 change of habitat and loss of vertical balance, we should then have to 

 consider the twisting round of the downward turned eye and the absence 

 of pigment cells on the downward side. 



As to the change of the eye, it may be said (1) that this has gradually 

 resulted from the efforts of the fish to continue to use the lower eye, a 

 possible interpretation if acquired characters can be transmitted. (2) It 

 may be said by those who do not believe in " use inheritance," that the 

 twisting round of the lower eye is not a result of a transmitted growth- 

 tendency at all, but is wrought out by effort in each generation de 

 novo. But young turbot and brill have nearly completed the twisting 

 round of the lower eye long before they have abandoned their pelagic 

 habit. (3) It may be said that the twisting round of the lower eye 

 arose as a germinal variation, apart from any direct influence of function 

 or environment, and that it has been retained and strengthened in the 

 usual course of natural selection. 



Again, as to the absence of chromatophores, it may be supposed that 

 this also is a useful adaptive character persistent as the result of 

 selection. But, apart perhaps from economy, it is not evident in what 

 the advantage consists. It seems possible that the under surface is 

 unpigmented because it is shaded ; and Mr. J. T. Cunningham, who has 

 devoted special attention to the problem of flat-fishes, has proved 

 experimentally that artificial illumination of the lower sides by means 

 of a mirror induces the development of pigment cells. It must be 

 noted, however, that pigmentation of both sides occurs also as a natural 

 variation, and is then usually associated with structural deformity. 



SURVEY OF FISHES. 



(See Table, pp. 528-529). 



Elasmobranchii. Cartilaginous Fishes. 



Sharks and skates represent two very distinct types 

 included in this order. They are voracious carnivorous 

 fishes. The scales are placoid. There is no cover over 

 the (5-7) gill-apertures; anterior to these there is often 

 a spiracle, — the first gill-cleft, — with a rudimentary gill. 

 The gill-clefts are separated by complete septa. The 

 fins are large. The skeleton is mostly cartilaginous. The 

 tail is asymmetrical or heterocercal. The mouth extends 

 transversely on the under side of the head. The nostrils 

 are also ventral. A spiral fold extends along the internal 

 wall of the large intestine. Into the terminal chamber (or 

 cloaca) of the gut, the genital and urinary ducts also open. 



