314 THE SEA SHORE 



coloured rocks, weeds, and corals ; and when they do occur it is 

 probable that they serve principally as a means by which the 

 brightly coloured sex usually the male attracts its mate. We 

 say ' usually the male,' but why so ? Because the female requires 

 the protection of a more sombre colour in order that she may with 

 safety deposit her spawn for the perpetuation of her species. Again, 

 the male referred to needs the assistance of his gaudy coat only 

 during the breeding season, hence we find that he assumes the 

 bright colours as a wedding garment, to be cast off when the breed- 

 ing season is over. 



This leads us to the subject of changeability of colours in the 

 same individual. That such changes do occur is well known, and 

 it is still more remarkable that they are produced in rapid succes- 

 sion, apparently at the will of the fish concerned ; for its tints will 

 vary as it moves from place to place so as to always harmonise 

 with the surroundings, and also in response to other conditions. 

 The mechanism by which such variations are produced has also 

 been studied and explained : The colouring matter is held in little 

 vesicles beneath the skin, and these vesicles are capable of being com- 

 pressed by muscles quite under the control of the fish. When they 

 are globular in form the contained pigment appears dark, but when 

 they are flattened by muscular compression, the pigment is spread 

 over a much larger area, and thus greatly reduced in depth of tint. 



As with all vertebrates, the central axis of the internal skeleton 

 of a fish consists of the backbone and the skull. The structure of 

 the latter is so complicated, and its description so full of techni- 

 calities, that we deem it advisable to pass it over in a work like this 

 where the scope is so large in proportion to the space available ; and 

 this we do with reluctance, because the detailed study of the skull 

 is of real importance to those who would thoroughly understand 

 the principles of classification. 



The backbone consists of a variable number of cylindrical 

 vertebrae, united end to end to form a continuous column, both the 

 anterior and posterior faces of each being concave. On the dorsal 

 surface of each vertebra there is a V-shaped arch, surmounted by 

 a spine, the former serving to protect the spinal cord, and the 

 latter giving attachment to the muscles of the back. Some of the 

 vertebrae are also provided with processes for the attachment of 

 the ribs, and those of the tail possess an arch and a spine on the 

 ventral as well as on the dorsal side. 



It has already been shown that the pectoral fins are jointed to 



