246 The Geographical Distribution of Fishes 



are derived from littoral types, the changes in structure being 

 due to degeneration of the osseous and muscular systems and 

 of structures not needed in deep-sea life. 



The fishes of the great depths are soft in substance, some of 

 them blind, some of them with very large eyes, all black in 

 color, and very many are provided with luminous spots or areas. 

 A large body of species of fishes are semi-bathybial, inhabiting 

 depths of 20 to 100 fathoms, showing many of the characters 

 of shore fishes, but far more widely distributed. Many of the 

 remarkable cases of wide distribution of type belong to this 

 class. In moderate depths red colors are very common, cor- 

 responding to the zone of red algae, and the colors in both 



FIG. 175. Sting-ray, Dasyatis sabina Le Sueur. Galveston. 



cases are perhaps determined from the fact that the red rays 

 of light are the least refrangible. 



A certain number of species are both marine and fresh water, 

 inhabiting estuaries and brackish waters, while some more 

 strictly marine ascend the rivers to spawn. In none of these 

 cases can any hard and fast line be drawn, and some groups 

 which are shore fishes in one region will be represented by semi- 

 bathybial or fluviatile forms in another.* 



* The dragonets (Callionymus) arc shore fishes of the shallowest waters in 

 Europe and Asia, but inhabit considerable depths in tropical America. The 

 sea- robins (Prionotus) are shore fishes in Massachusetts, semi-bathybial fishes 

 at Panama. Often Arctic shore fishes become semi-bathybial in the Temper- 

 ate Zone, living in water of a given temperature. A long period of cold 

 weather will sometimes bring such to the surface. 



