MID-GUT OF VEETEBRATA, 



561 



rudiment, is au adaptation to the functions required of it by tlie 

 ingesta. 



In tlie Amphibia the simple condition of the mid-gut is very 

 rarely permanent (Proteus). It generally forms, as it does also in 

 Reptiles, a longer tube, and, consequently, a 

 number of coils. (Fig. 317, i.) In the Ophidii 

 these are least, in the Chelonii they are con- 

 siderably, and in the Crocodilini they are still 

 more developed. The mid-gut is very greatly 

 elongated in the larva3 of the anourous Amphibia, 

 where this portion forms a long loop arranged 

 in spiral coils. It is reduced when the mode of 

 feeding is changed during the final stages of 

 larval life, and this leads to an abbreviation of 

 the length of the enteron. 



The length of the mid-gut in Birds also varies 

 very greatly according to the characters of their 

 food. It is arranged in loops, the first of which 

 (duodenal loop) is the best developed, and always 

 contains the pancreas. 



The mid-gut of Mammals is seen no less dis- 

 tinctly to vary in length according to the kind of 

 food that is eaten ; so that there are different 

 conditions in Carnivorous and Herbivorous forms. 



The surface of the mid-gut is increased by 

 various arrangements of its mucous membrane, 

 as well as by its increase in length. In the lower 

 groups there are coarser folds (spiral valve of 

 the Selachii), but in the Amphibia and Reptilia 

 by far the most common arrangements are fine 

 longitudinal folds of the mucous membrane. These obtain also in 

 the Birds, but in them they generally form unequal elevations, and 

 may be united by transverse lines. Fine folds arranged in zigzag 

 lines are seen in the Amphibia and Eeptilia, and are found also on 

 the mid-gut of Birds. In Mammals, these longitudinal folds of 

 the mucous membrane are commonly found in the Cetacea ; but in 

 most of the other Mammalia the mucous membrane is smooth, or 

 raised up into transverse folds, which are very generally beset with 

 villi. When the folds are feeble we find that these villi are greatly 

 developed in Birds also, while when the folds are present the villi 

 are merely smaller elevations. 



Fig. 317. Enteric 

 canal of Menobran- 

 chus lateralis. 

 2^ Commencement of 

 the fore-gut with the 

 Pharynx, oe ffiso- 

 phagus. V Stomach. 

 i Mid-gnt. r Hind- 

 gut. 



Hind-gut. 



§ 420. 



The end- or hind-gut is the smallest of all in the lower 

 divisions, and is merely represented by a short and somewhat wider 



2 



