100 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 alge, and the colors in both 
Fie. 72.—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) are 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. 
