48 ZOOLOGY. 



deposited in water in a mass. They are destitute of shells. 

 Each ovum consists of a black yelk, surrounded by a 

 gelatinous "white." The ova are impregnated by the 

 male immediately after exclusion. If a quantity of the 

 spawn of the common frog is placed in water in a glass 

 vessel, the following phenomena will be observed. After 

 a time small fish-like animals will proceed from the dark 

 specks of yelk. These, if viewed by a microscope, will 

 be seen to be destitute of feet, to possess external gills in 

 the form of branched tufts, and a long tail, surrounded 

 by a "median fin," unsupported by rays. After a time the 

 ijflls decay, the hind feet are developed, then the fore- 

 feet. The tail is gradually absorbed into the body, and 

 finally disappears altogether. In the newts the fore-feet 

 make their appearance first, and the tail is retained. 



CLASS Y.Pisces. 



58. The Pisces or fishes (Lat. piscis, a fish) form the last 

 class of vertebrate animals. These animals live entirely 

 in water, and for this mode of life they are admirably 

 adapted, the shape of their bodies, and the arrangement 

 of the scales with which they are covered, presenting 

 the least possible resistance to the medium through which 

 they move. 



Fishes form the second class of the section IchtJiyop- 

 sida. They agree with the Amphibia in breathing by 

 gills which are permanent; in being destitute of the 

 membranes ainnion and allantois in the embryonic state ; 

 in possessing cold blood, and in having oval, nucleated, 

 red blood-corpuscles. They are also, like the Amphibia, 

 oviparous animals. 



They differ from the Amphibia in being covered with 

 scales instead of having a naked skin ; in possessing a 

 heart with only a single auricle and a ventricle. In the 

 fishes also, the nasal cavities do not open behind into the 

 pharynx, as is the case with all the higher Vertebrata. 



