96 GENERAL PRINCIPLES OF ZOOLOGY 



cloaca (0). In the more highly organized animals there are accessory 

 structures connected with the alimentary canal. Into the mouth empty 

 the salivary glands; into the first part of the small intestine, close behind 

 the stomach, the liver (e) and the pancreas (g) (or a single glandular 

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

 creatic juice, the h e pat o pancreas}. 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 

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

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

 tissues by the blood and lymph vessels. Digestion is effected by fluid ferments 

 (enzymes], substances which by their presence can produce definite chemical 

 changes without, apparently, being altered themselves. Thus the pepsin 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 pan- 

 creas has the same effect in alkaline media; the steapsin of the bile saponifies 

 fats and makes them resorbable, while the ptyalin of the saliva converts starch 

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

 assimilated; that is, so altered and appropriated 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 alimentary canal being 

 chiefly resorbtive. 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. When we speak of the liver of a crustacean, 

 spider or mollusc, we must remember that this organ not only dissolves fat, and 

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

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

 ticles being taken into the cell. A similar condition obtains in many coelenter- 

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

 a true digestion in the archenteron, the walls of which secrete digestive fluids. 



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

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

 thus formed being called the splanchnopleure. 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 moved about inde- 

 pendently of the body musculature. When these splanchnopleuric muscles are 

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

 the cilia which may cover the digestive epithelium. 



The length of the digestive tract is chiefly influenced by the kind of food. In 

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

 carnivores, the former having a very long and consequently convoluted digestive 

 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 differences between carnivor- 

 ous and plant-eating beetles. 



II. Respiratory Organs. 



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

 animal must obtain in exchange for the carbon dioxide formed in the 



