112 GENERAL GliOWTH. 



{forming a cavity, known as the branchial cavity, in which the gills 

 are placed. Each branchial cavity at first opens by a separate widish 

 pore behind (fig. 80), and in Dactylethra both branchial apertures 

 iire preserved (Huxley). In the larva of Bombinator, and it would 

 seem also that of Alytes and Pelodytes, the original widish openings 

 1 of the two branchial chambers meet together in the ventral line, 



and form a single bran- 

 chial opening or spiracle. 

 In most other forms, i.e. 

 Rana, Bufo, Pelobates, 

 letc, the two branchial 

 /chambers become united 

 by a transverse canal, and 



^ „, „ T^ .the opening of the right 



Fig. 81. Tadpole of Bombinator from the ' i +1, Vv 11 



VENTRAL SIDE, WITH THE ABDOMINAL WALL REMOVED. iSaCK tXlCn VaniSlieS, Willie 



(A-fter Gotte.) 'that of the left remains 



Behind the mouth are placed the two suckers, aS the single unsymme- 

 and hehind these are seen the gills projecting i ^^-^^j spiracle. In breath- 

 through the spiracles. ^ . , ^ ^ • , i 



ing the water is taken m 



at the mouth, passes through the branchial clefts into the branchial 

 cavities, and is thence carried out by the spiracle. 



Immediately after the formation of the branchial cavities, the 

 original external gills atrophy, but in their place fresh gills, usually 

 called internal gills, appear on the outer side of the middle region of 

 the four branchial arches. 



There is a single row of these on the first and fourth branchial 

 arches, and two rows on the second and third. In addition to these 

 gills, which are vascular processes of the mesoblast, covered, according 

 to Gotte, with an epiblastic (?) epithelium, branchial processes appear 

 on the hypoblastic walls of the three branchial clefts. The last-named 

 branchial processes would appear to be homologous with the gills of 

 Lampreys. In Dactylethra no other gills bufthese are formed (Parker). 



The mouth, even before the tadpole begins to feed, acquires a 

 transversely oval form (fig. 81), and becomes armed with provisional 

 structures in the form of a horny beak and teeth, which are in use 

 during larval life. 



The beak is formed of a pair of horny plates moulded on the upper and 

 lower pairs of labial cartilages. The upper valve of the beak is the larger 

 of the two, and covers the lower. The beak is surrounded by a projecting 

 lip formed of a circular fold of skin, the free edge of which is covered by 

 papillae. Between the papillae and the beak rows of horny teeth are 

 placed on the inner surface of the lip. There are usually two rows of these 

 on the upper side, the inner one not continuous across the middle line, and 

 three or four rows on the lower side, the inner one or two divided into two 

 lateral parts. 



As the tadpole attains its full development, the suctorial organs 

 behind the mouth gradually atrophy. The alimentary canal, which 

 is (fig. 81) at first short, rapidly elongates, and fills up with its 



