PEOGS AND TADPOLES. 267 



water. A sucker is developed, beneath, where the mouth will 

 appear, by means of which the tadpole can attach itself to weeds 

 or other objects in the water. A few days after hatching, the 

 mouth is opened, and it now feeds on vegetable matter the 

 previous development having been sustained from the reserve 

 food in the white and yolk. 



This brings us to the fourth stage. Again we find many 

 animals which persist in this stage, having external gills, and 

 although perfect in all respects must be regarded simply as 

 Tadpoles. The Tadpole, however, does not long remain in this 

 condition, soon four gill-slits make their appearance on each side 

 of the neck. These are openings leading from the outside into 

 the throat, and are developed between the four branchial arches. 

 The sides of these slits become folded to form the internal gills, 

 and as this process takes place the external gills wither away 

 and the Tadpole becomes a true fish. 



By a fish, of course, we do not mean any animal which hap- 

 pens to live in the water, but an animal possessing certain 

 specific characters, namely: — a backbone; breathing by gills; 

 the course of the circulation of the blood. 



The circulation of the blood is very characteristic in the fish. 

 The heart pumps the blood directly into the gills, where it is 

 purified, it is then collected into the artries and distributed over 

 the body, the mammalian method being for the heart to directly 

 distribute the blood over the body. The tadpole exactly corres- 

 ponds with the fish, its heart pumps the blood at first into the 

 external gills, where it is aerated, and a little later into the in- 

 ternal gills. The gill-slits are now no longer exposed, a fold of 

 skin grows from the sides of the head, and proceeding backwards, 

 forms a chamber into which the gills open externally, and which 

 itself opens by an aperture on the left side of the tail. 



The tadpole feeds very freely and as a consequence increases 

 very much in size. Its tail, too, lengthens considerably and 

 becomes a powerful swimming organ. "While still in the fish- 

 stage, the limbs begin to appear. The hind limbs are first 

 apparent and are formed on each side of the root of the tail. The 

 fore-limbs are formed inside the gill chamber and have to force 



