128 GENERAL PRINCIPLES OF ZOOLOGY. 



sometimes occur glands which form a fetid secretion the 

 anal glands. 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 carni- 

 vores, the former having a very long and consequently 

 tortuous digestive tract. That of a carnivore is somewhat 

 less than 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 carnivorous 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 

 carbonic acid formed in the tissues is derived either from 

 the air or from the water, according as the animal is a land- 

 or a water-dweller. Less frequently it is the case that 

 water-dwellers breathe air, and hence are compelled, from 

 time to time, to rise to the surface of the water for a sup- 

 ply of air; this is true for the great marine mammals, and 

 for many insects, spiders, and snails which are found in 

 fresh water. Air- and water-breathing takes place exclu- 

 sively through the skin, so long as this is soft and readily 

 permeable, and so long as no higher development of organ- 

 ization necessitates a more active interchange of material. 

 If, on the other hand, the demand for oxygen is greater, 

 other more special breathing-organs are found gills for 

 water-breathing, lungs and tracheae for air-breathing, in 

 addition to which the skin functions as an accessory organ 

 of more or less importance. 



Gills. The gills are usually thin-walled areas of the 

 skin which are particularly well supplied with blood-ves- 

 sels, and where richly branched tuftlike projections or broad 

 leaves have grown out, thus furnishing the largest possible 

 surface for the interchange of gases; these lie in such a 

 position as to be most exposed to fresh water; in the 



