96 GENERAL rRlXClTLES OF ZOOLOGY 



cloaca (('). In the more liighlv organized animals (here are accessor)' 

 structures connected \vilh the alimentary canal. Into the mouth empty 

 the salii\irv t;/cj;;(/.\-; iiilo the lirst part of the small intestine, close beiiind 

 the stomach, the //■;•<■/• (<■) and the pancreas (g) (or a single glandular 

 apparatus, whose secretion combines the characters of gall and of pan- 

 creatic juice, the licpalopancrcas). Finally, in the hind-gut there sometimes 

 occur glands which form a fetid secretion — the anal glands. 



Digestive Functions. — Besides the comminution of the food which is often 

 necessarv, the alimentary canal has (i) to digest the food; that is to convert it 

 into a solution; and (2) to rcsorb the digested food, that is to forward it to the 

 tissues bv the blood and Ivmph vessels. Digestion is effected by tluid ferments 

 (enzvmes), substances which by their presence can produce delinite chemical 

 changes without, apparently, being altered themselves. Thus the jiejisin from 

 the gastric glands of the vertebrates, in the presence of hydrochloric acid, can 

 convert the proteid of the food into the soluble peptone; the trypsin of the ]ian- 

 creas has the same effect in alkaline media; the steapsin of the bile sa]ionihes 

 fats and makes them resorbable, while the ptyalin of the sali\a converts starch 

 into sugar. The resorbcd substances are distributed to the tissues and are here 

 assimilated; that is, so altered and appiropriated that it becomes an integral part 

 of the living, functioning structures — muscles, nerves, cells, etc. 



In the vertebrates there is a division of labor, the glands functioning exclu- 

 sively in furnishing the digestive fluids, the walls of the alimcntarv canal being 

 chiet3y rcsorbtive. In the invertebrates this distinction has not gone so far, so 

 that the transfer of names from the higher group may lead to misconceptions 

 as to the functions of the organs, ^^'hen we speak of the liver of a crustacean, 

 Sjiider or mollusc, we must remember that this organ not onlv dissolves fat, ancl 

 proteids and cellulose as well, but that it plays an im]iortant part in the resorb- 

 tion of nourishment. In the protozoa there is a cellular digestion, food ]iar- 

 ticles being taken into the cell. A similar condition obtains in manv cwlentcr- 

 ates, the individual entodermal cells eating the food particles; but there is also 

 a true digestion in the archentcron, the walls of which secrete digestive fhiids. 



Even when the digestive tract has but little dilTcrentiation of its parts, it 

 usually has a mesodermal layer added to its entodermal lining, the whole wall 

 thus formed being called the splanelinopleiirc. The mesodermal additions take 

 the form, not only of connective tissue but muscles as well (usually smooth, 

 rarely of the striped variety). These muscles are important in effecting the 

 {peristaltic) motion by which the contents of the canal are mo\ed about inde- 

 pendently of the body musculature. When these splanchnojileuric muscles are 

 absent, the movement is caused by the contraction of the bodv muscles, or by 

 the cilia which mav cover the digestive epithelium. 



The length of the digestive tract is chielly inlhienced bv the kind of food. In 

 many groups of animals there is found a difference liotween herbivores and 

 carnivores, the former having a \ ery Ion:; and consequently convoluted digesli\c 

 tract. That of a carnivore is about four or five times the length of the body, 

 while in an herbivorous ungulate, on the other hand, it is twenty to twenty- 

 eight times. Similar, though not so great, are the dillereiu'es between carnixo'r- 

 ous and plant-eating beetles. 



II. Respiratory Organs. 

 Sources of the Oxygen used in Breathing. — The oxygen which each 

 animal must obtain in exchange fiu' the carbon dioxide formed in the 



