312 COMPARATIVE ANATOMY 



to prevent the food from passing along the canal in the wrong 

 direction, but also to retain it within the same portion of the 

 canal for a certain time. There is also a sphincter muscle at the 

 anus. 



In almost all cases the alimentary canal becomes more or less 

 coiled, and thus presents a greater surface for absorption. As a 

 general rule, it is relatively longer in herbivorous than in carni- 

 vorous animals. A considerable increase of surface also commonly 

 results from the elevation of the mucous membrane to form folds, 

 villi, and papillae. 



Certain appendages are also present in connection with the 

 alimentary canal. These are all developed primarily from the 

 endoderm and are thus of epithelial origin : mesodermic elements 

 are added to them secondarily. Whether serving as glands or 

 not, they all arise in the same manner as glands. 



Beginning from the mouth the following appendicular organs 

 of the alimentary canal may be distinguished (Fig. 227) : 



(1) Mucous and salivary glands. 



(2) The thyroid. 



(3) The thymus. 



(4) The swim-bladder or lungs. 



(5) The liver. 



(6) The pancreas. 



In addition to these, gastric and intestinal glands are embedded 

 in the wall of the gut. 



Oral Cavity. 



In Amphioxus and Cyclostomes the aperture of the mouth is 

 surrounded by an oral hood or funnel supported by skeletal parts, 

 which, in the former and in Myxinoids, is edged with tentacles or 

 cirri : all other Vertebrates are provided with an upper and a 

 lower jaw ^ 



Definite lips provided with muscles first appear in Mammals, 

 and are very varied in form. The space between them and the 

 jaws is spoken of as the vestibulum oris ; this may become extended 

 on either side to form cheek-pouches, which serve as food reservoirs 

 (many Monkeys and Rodents). 2 The lips, together with the 

 cheeks and mobile tongue, are important in suction, as well as 

 in articulate speech (Man). Monotremes are the only Mammals 

 in which they are wanting : in them the jaws are covered by a 



1 The mouth of the Lamprey serves as a suctorial organ for attaching the 

 animal to foreign objects. The larva; of Lepidosteus, 1'olypterus, Lepidosiren, 

 Protopterus, and Anura are temporarily provided with suctorial organs. 



2 Cheek -pouches, opening externally anil lined by hair, occur amongst 

 Rodents (Geomyidse). 



