DEVELOPMENT. 149 



Dog-fishes, which lay large eggs enclosed in a horny shell, a true 

 copulation and an internal fertilization of the egg takes place. It is 

 worthy of note that in a few exceptional cases the male undertakes 

 the charge of the brood (Hippocampus, Cottus, Gasterosteus}. 



The embryonic development of the fishes is principally dis- 

 tinguished from that of the higher Vertebrates by the fact that 

 neither amnion nor allantois are developed. Both the small eggs of 

 the Teleosteans, which are provided with a micropyle, and the large 

 eggs of the Plagiostomes, which are surrounded by a hard horny 

 case, contain a large quantity of food yolk, and undergo a partial 

 segmentation. The eggs of Amphioxus and of the Cyclostomes, 

 however, undergo a total segmentation. As a rule the young fishes 

 leave the egg-membranes tolerably early, with more or less distinct 

 remains of the yolk-sac, which is by this time completely taken up 

 into the interior of the body, but projects externally, like a hernia. 

 Although the body-form of the just-hatched fish differs essentially 

 from that of the adult animal, yet no true 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 Plagio- 

 stomes, live almost entirely in the sea ; others, as the Cyprinoidei 

 and Esocidce, 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 spelceus). 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. Certain East Indian fresh-water fish (Anabas scandens), 

 whose upper pharyngeal bones are hollowed out in a labyrinthine 

 manner and form a multicellular reservoir for water, are capable 

 of living longer out of water than any fish except the Dipnoi. There 

 are even fishes which can fly (Exoccetus, Dactylopterus). 



Fishes are of great importance to our knowledge of the develop- 

 ment of animal life on the earth owing to the frequent appearance of 

 their fossil remains in all geological periods. In the palaeozoic 

 formations very singular fish-forms, as the Cephalaspidce (Cepalaspis, 

 Coccosteus, Pterichthys), constitute the oldest representatives of the 



