STRUCTURE OF ORGANISMS 467 



canal comprised in these three regions. In some theforegut and hind-gut 

 are relatively short and the mid-gut long, but in arthropods the mid-gut 

 is only a short region between the stomach and intestine. In chordates 

 the mid-gut is very long, the stomodeum giving rise to the mouth and the 

 proctodeum to the posterior part of the rectum. 



In the phyla possessing an alimentary canal has been observed the 

 gradual appearance of specialization, shown by the setting aside and 

 modification of particular regions for the performance of certain functions. 

 For more effective functioning, various types of glands secreting different 

 digestive enzymes have appeared. The absorptive surface has been 

 increased by an increase in the length of the canal, by the formation of 

 folds and villi, and by the production of blind sacs called caeca the cavities 

 of which open into the lumen of the canal. A correspondence can gen- 

 erally be traced between the length of the canal and the character of the 

 food, carnivorous forms having a short alimentary canal and herbivorous 

 ones a longer and more capacious one. In the vertebrates this specializa- 

 tion of the canal reaches its highest development; the conditions there 

 have been outlined in Chap. L. 



476. Glands. — One of the functions of epithelial cells is the produc- 

 tion of secretions or excretions. Epithelia of which this is the most 

 prominent function are termed glandular. The secretion may be watery, 

 in which case it is termed serous; or it may be thicker and contain 

 mucin, when it is called mucous. The secretion may serve to moisten 

 the surface of the body and prevent drying, as does that of the glands of 

 the amphibian skin; it may prevent contact with water and the entrance 

 of infectious organism, which is true of that of the skin glands of the 

 fishes; or it may assist in temperature regulation, as in the case of perspi- 

 ration in mammals. Other functions of such secretions are to lubricate 

 internal surfaces or to aid in digestion, as in the cases of many glands 

 connected with the digestive system. 



One gland cell may function alone or many such cells may be grouped 

 to form a gland. Glands are usually removed from the surface, and the 

 secretion which is poured into the cavity, or lumen, of the gland reaches 

 the surface through a tubular duct. In the case of oil glands and milk 

 glands, the secretion is not passed out of the gland cell leaving the latter 

 intact, as in most glands, but the cells themselves are broken down and 

 passed into the lumen and out through the duct as part of the secre- 

 tion, new cells appearing to take their places. Glands are classified, 

 according to their shape, as tubular or alveolar and, according to the 

 number of parts into which they are divided, as simple or compound 

 (Fig. 351). 



477. Respiratory System. — The respiratory system is one of the 

 latest systems to appear. In protozoans respiration takes place through 

 the surface of the cell and expiration is supposed to be aided, at least, by 



