150 THE FROG. 



ably later period. This early appearance of the proctodeeal 

 opening is perhaps to be associated with the early formation of 

 the kidneys, which are already present, and have ducts opening 

 into the hinder end of the mesenteron (Figs. 69, KA, and 74, KP, 

 KA), shortly before the time of hatching of the tadpole. 



The development of the several regions of the alimentary 

 canal, and the structures arising in connection with them, will 

 now be described in more detail, with the exception of the gill- 

 clefts and gills, which form the subject of the next section of 

 tliis chapter. 



2. The Lips. 



The mouth of the tadpole is very small compared with that 

 of the frog (cf. Figs. 85 and 86, p. 193). It is surrounded by 

 prominent frill-like lips, which form a short conical proboscis 

 (Figs. 83 and 85, LI, LJ). The inner surfaces of the lips bear 

 rows of minute teeth, and at the bottom of the funnel, sepa- 

 rating the proboscis, or labial cavity, from the buccal cavity, is 

 the beak, formed by the two powerful horny jaws (Fig. 65, j). 



There are two lips, upper and lower, which are continuous 

 with each other at the angles of the mouth, so as to completely 

 surround the opening. The upper lip (Figs. 65 and 83, Li) 

 is a crescentic fold of integument bounding the labial cavity in 

 front ; it is smaller and less mobile than the lower lip, and 

 bears along its free edge a row of minute horny teeth. The 

 lower lip (Figs. 65 and 83, Lj) is both longer and deeper than 

 the upper ; it is also softer and much more mobile. It is 

 separated behind by a well-marked transverse groove from the 

 under surface of the head, and is produced at its free edge into 

 a series of small fleshy papillae. These papillae, which are pro- 

 bably tactile in function, are more numerous at the angles of 

 the mouth, where they are arranged in groups. 



The inner surfaces of the lips, between their free edges and 

 the beak, bear transverse ridges or folds, which support along 

 their crests comb-like rows of minute black horny teeth. Of 

 these rows, the upper lip, in addition to the row round the 

 margin already mentioned, has three incomplete rows, interrupted 

 in the middle by a considerable interval. The lower lip bears 

 four similar but complete rows of teeth. 



Each of these teeth is formed by modification of a single 



