386 ORGANS OF DIGESTION. 



food which consists of everything organized in the rich element 

 they inhabit. The rugse of the mucous coat, which are some- 

 times longitudinal or transverse, or reticulate on the intestine 

 of osseous fishes, form a remarkable continuous, elevated, spi- 

 ral fold in the plagiostome chondropterygii, which winds round 

 the interior of the canal, from the duodenum to the rectum. 

 The rectum here terminates, as in other oviparous vertebrated 

 animals, in a common cloaca, and has behind it in both sexes 

 the opening, single or double, of the genital organs, the pos- 

 terior part being occupied by the urinary passages. On the 

 sides of the anus, in most of the cartilaginous and many of the 

 osseous fishes, as in some aquatic reptiles, there are two oblique 

 valvular openings, leading externally from the cavity of the 

 peritoneum, and affording an easy exit to matters passing in 

 that direction, but impeding their entrance from without. The 

 peritoneum lining the interior parietes of the abdomen, and 

 extending, like the pericardium above, over tendinous fibres 

 of the diaphragm, has often the shining silvery lustre and 

 white colour of the rete mucosum without ; the mesentery is 

 generally more developed in the osseous than in the cartilagi- 

 nous fishes, and folds of the peritoneum, charged with adipose 

 substance, sometimes hang from the outer margin of the in- 

 testine, forming a rudimentary epiloon. This imperfect con- 

 dition of the mesentery in the lowest fishes, where the intes- 

 tine is connected only by blood-vessels, approximates them to 

 the invertebrata, and the same is often found as an abnormal 

 character in man. The simple, straight, cylindrical form of 

 the whole alimentary tube in many of the lowest fishes like- 

 wise assimilates them to the earlier conditions of the human 

 embryo, as also the follicular character of their principal chy- 

 lopoietic glands. As the umbilical vesicle in the osseous fishes 

 passes entirely into the abdomen, to complete the intestine 

 and the abdominal parietes by its mucous and serous coats, 

 there can be no umbilical mark left in the adult, and there is 

 no allantois communicating with the cloaca in the young, as 

 they require no placental nutriment. 



XX. Amphibia. The amphibious animals, like the fishes, 

 are mostly predaceous in their habits, and swallow their prey 

 entire, having loose and feeble articulations of the jaws, and 

 sharp, slender, prehensile teeth, ill adapted for mastication. 

 The teeth are placed sometimes, like those of fishes, in se- 

 veral rows on the palatine and both maxillary bones, as in 



