232 



ESSENTIALS OF ZOOLOGY 



The heart of the tadpole in the early stages is fish-like. 

 It consists in succession of a sinus venosus, an auricle, a 

 ventricle, and a conus arteriosus. The latter is put into com- 

 munication with the external gills by four afferent branchial 

 vessels which develop from a common trunk in regular succes- 

 sion, the first being the first formed. The blood is led in 

 efferent branchials to the two dorsal aortae. The aortae send 

 the blood into the head by carotid arteries, and, uniting behind 

 the gills, carry the blood to the posterior end of the larva. 



Cut.a Glom.a. 



Carotid a 



Coel.rn.es. a. 

 orta 



Lingual a. 



Pulmon. a, and v. 

 Post, vena cava 



Pulmon. a. 



Carotid a. 



Lingual a 



FIG. 119. Lateral diagrammatic views of arterial circulation of 

 tadpole and frog. 



The venous blood is carried back to the heart by a sub-intestinal 

 vein, and by cardinal veins which form a Cuvierian vein on 

 each side to enter the sinus venosus. The circulation, there- 

 fore, is that of a fish. With the appearance of the internal 

 gills the blood is more and more sent to them, and the 

 circulation to the external gills is correspondingly reduced. 



The length of the tadpole stage of the life of the frog 

 depends on circumstances, and if means be taken to prevent 

 the tadpole gaining the surface it may be prolonged, large 

 tadpoles resulting. In some cases, as that of Axolotl, the call 

 to the adult state may be postponed for an indefinite number 

 of generations, the creature reproducing in the meantime as 

 a larva. In the case of the frog the change usually takes 

 place after about three months. 



