CHAPTER V 

 ALIMENTATION 



The energy-producing substances of an animal's diet consist 

 of chemically complex compounds, viz., proteins, carbohydrates, 

 and fats, each of which undergoes digestion before being absorbed. 

 Water, simple sugars, and some inorganic salts, such as sodium 

 chloride, taken with food do not require digestion. In the higher 

 forms, the organs of digestion include, in addition to the ali- 

 mentary canal in which digestion and absorption take place, a 

 number of glands, such as the oral (salivary) glands, the pancreas, 

 and the liver, though the last in vertebrates does not produce a 

 digestive enzyme. 



Gastrovascular Cavity. — A simple kind of alimentary tract is 

 that of the common Hydra, a tube-shaped animal found in fresh 

 water, its length in different species varying from 2 to 60 mm. 

 The closed end of the tubular body is attached by a basal disk 

 to the substratum, which is usually a plant; the opposite free end 

 is open and serves as a mouth. The mouth is surrounded by 

 from six to ten long, narrow, tubular tentacles. The body wall 

 consists of two well-defined cellular layers, an outer ectoderm and 

 an inner endoderm, with some indication of a third layer between 

 them. The third layer, known as the mesoglea, is noncellular and 

 of jellylike consistency. Since only two embryonic germ layers, 

 the ectoderm and endoderm, are developed, the animal is said 

 to be diploblastic. Each tentacle likewise consists of an inner 

 layer of endoderm and an outer layer of ectoderm. The cavity 

 of the tube, the gastrovascular cavity, is the digestive cavity 

 (Fig. 51). 



Food captured by the tentacles passes through the mouth into 

 the gastrovascular cavity, where it is acted upon by a secretion of 

 the cells lining the cavity. Some of the food is digested in this 

 way; and some of it apparently is engulfed by the endodermal 

 cells to undergo intracellular digestion. The products of extra- 

 cellular digestion are absorbed by the endoderm and any undi- 



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