546 PISCES FISHES. 



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 (i) 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. 



CLASSIFICATION OF FISHES. 



Sub-Class I. ELASMOBRANCHII. Cartilaginous Fishes, 



e.g. Sharks and Skates. 



Voracious carnivorous fishes, with cartilaginous skeleton, 

 placoid scales, usually heterocercal tails, " claspers " on 

 the pelvic fins of the males. Except in Holocephali 

 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, and the gill-filaments are attached throughout their 

 length to the septa. The mouth extends transversely on 

 the under side of the head. The nostrils are also ventral. 

 There is no air-bladder. 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. The ventricle of the heart has a con- 

 tractile conus arteriosus. Fertilisation is internal. The 

 ova are few and large, i.e. with much yolk. Large egg- 

 purses are common, but some Elasmobranchs are viviparous. 



