92 CLASS PISCES. 



the young fishes leave the egg- membranes tolerably early, with 

 more or less distinct remains of the yolk-sac, which still projects 

 externally, like a hernia. Although the body form of the just- 

 hatched fish differs essentially from that of the adult animal, no 

 sudden metamorphosis takes place save in a few exceptional cases. 



Most fishes live in the sea, and the number of their species and 

 genera increases as we approach the equator. But they are not 

 all exclusively confined to fresh or salt water. Many, as the 

 Plagiostomes, live almost entirely in the sea ; others, as the 

 Cyprinidae and Esocidae, are confined to fresh water, but there 

 are also fish which periodically change their habitat, especially 

 at spawning time. Some fish live in subterranean waters, and 

 are blind, like the inhabitants of caves (Amblyopsis spelaeus). 

 Few fish are able to live any length of time out of water ; as a 

 rule the wider the gill-slits the quicker does the fish die on dry 

 land. Fishes with narrow gill-slits (Eels) possess an uncommon 

 tenacity of life out of water. According to Hancock, a species of 

 Doras migrates in great shoals over the surface of the ground 

 from one piece of water to another, Except the Dipnoi, certain 

 East Indian freshwater fish, the upper pharyngeal bones of 

 which are hollowed out into the form of a labyrinth (Fig. 38) 

 and form a multicellular reservoir for water, are capable of living 

 the longest time out of water (Anabas scandens). There are 

 even fishes which can float through the air (Exocoetus, 

 Dactylopterus) . 



Marine fishes may be distinguished into shore fishes, pelagic 

 fishes, and deep-sea fishes, which, as in the case of marine and 

 fresh-water fishes, graduate into one another. Shore fishes live 

 near the surface, and do not descend to any great depth ; they 

 are comparatively restricted in range. Pelagic fishes inhabit 

 the surface waters of the ocean, where they usually spawn, 

 though some visit the shores for this purpose ; they are usually 

 strong swimmers and wide ranging, but a few (e.g. Hippocampus, 

 Antennarius, etc.) are poor swimmers, and infest floating sea- 

 weed, or drift on the surface. Some pelagic fishes come to the 

 surface at night only, descending in the daytime to a consider- 

 able depth (Brama, Sternoptychidae, Scopelus, Astronesthes). 

 The largest fishes belong to the pelagic fauna, e.g. Rhinodon, 

 Selache, Carcharodon, Myliobatidae, Thynnus, Xiphiidae, Ortha- 

 goriscus. The features of deep-sea fishes are referred to below. 



