270 



ZOOLOGY 



SECT. 



becoming separated by a slight constriction, is marked out as the 

 head : the other end is bluntly pointed and is the rudiment of 

 the tail (.). On the ventral surface of the root of the tail a procto- 

 dceum (pcdm.) appears and communicates with the archenteron. 



The head and tail become more distinctly marked off from the 

 trunk. A pit the stomodceum (J L, st. dm.) appears on the 

 antero-ventral surface of the head, and, immediately behind it, a 

 semilunar area with raised edges, the sucker (sk.). At each side of 

 the head two branched processes appear; they are the external 



FIG "'886. Ran a temporaria. Stages in the life-history, from the newly -hatched Tadpoles (1) 

 to the young Frog (8). 2 a is a magnified view of 2. (From Mivart.) 



gills (br 1 ., fo* 2 .), and the regions from which they arise mark the 

 positions of the first and second branchial arches. 



The embryos are now hatched as tadpoles. They swim freely 

 in the water or adhere to weeds by means of their suckers 

 (Fig. 886, 1). They are still blind and mouthless, the stomodaeum 

 not having yet communicated with the archenteron. Soon a third 

 pair of external gills appears on the third branchial arch, and the 

 first two pairs increase greatly in size (#,# a ); the stomodseum joins 

 the archenteron, gill-slits are formed between the branchial arches, 



