ORGANS OF DIGESTION. 387 



the siren, or more serpent-like in single rows on the palatine 

 bones and both maxillae, as in the triton, or only on the pala- 

 tine bones and upper jaw, as in faefrog ; but they are want- 

 ing in both jaws in the toad and the pipa, although two small 

 transverse rows are seen behind the posterior nares in the 

 toad. The teeth have here the same osseous texture, thin 

 coat of enamel, and feeble superficial attachment to the jaws 

 as in most fishes, and these animals are almost as destitute 

 of salivary glands as the permanent tadpoles of the sea. The 

 long, free, and bifid tongue of the frog, covered with papillae 

 arid muciparous follicles, more nearly approaches to the 

 ordinary form of that organ in the serpents and many higher 

 reptiles than the short, thick, fleshy form of the tongue com- 

 mon to the perennibranchiate amphibia and the toad. The 

 strong muscular oesophagus, short, dilatable, and longitudi- 

 nally plicated within like that of fishes, leads to a narrow 

 elongated stomach, directed transversely from left to right, 

 generally with thick fleshy parietes, especially at the pyloric 

 portion, and covered above by the two lobes of a large liver, 

 which is always provided with a distinct and free gall- 

 bladder. 



The stomach is most lengthened and narrow in the tadpole 

 state of the higher amphibia, and in the adult forms of the 

 aquatic species, as in the lower fishes and in the embryo 

 condition of man, and these also exhibit the least distinction 

 between the small intestine and the colon. In the young 

 tadpole (Fig. 125. B. C.) of the common frog, which sucks 

 with a small circular mouth (B. C. a, a,} the soft animal and 

 vegetable matter of our fresh water ponds, the stomach (c, 

 d,) is narrow and elongated, and the intestine (B. c, d, C. e, 

 e,) of extraordinary length, and nearly equal throughout, is 

 coiled up in a spiral manner, distending the capacious abdomen 

 and perceptible through the transparent parietes. It is only 

 slightly enlarged near the anus, and is attached to the verte- 

 bral column by an entire mesentery, as in the adults of all 

 the amphibia. During the metamorphosis which so remark- 

 ably affects every internal system, and in which the soft and 

 mixed food of the tadpole is changed for more nutritious 

 aliment, as snails, worms, caterpillars, and similar creeping 

 animals, the stomach and the whole alimentary canal become 

 gradually shortened in their proportions, and their divi- 



c c 2 



