136 



GENERAL GROWTH. 



form a single branchial opening or spiracle. In most other 

 forms, i. e. Rana, Bufo, Pelobates, etc., the two branchial chambers 

 become united by a transverse canal, and the opening of the 

 right sack then vanishes, while that of the left remains as the 

 single unsymmetrical spiracle. In breathing the water is taken 

 in 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 ap- 

 pear to be homolo- 

 gous with the gills 

 of Lampreys. In 

 Dactylethra no 



other gills but these 

 are formed (Parker). 



The mouth, even 



before the tadpole begins to feed, acquires a transversely oval 

 form (fig. Si), and becomes armed with provisional structures in 

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

 larval life. 



FIG. 81. TADPOLE UK BOMBINATOK FROM THE 



VENTRAL SIDE, WITH THE ABDOMINAL WALL REMOVED. 



(After Gotte.) 



Behind the mouth are placed the two suckers, and 

 behind these are seen the gills projecting through the 

 spiracles. 



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. 



