MORPHOLOGY OF ANIMALS 



Digestive Cells and Glands. In the simplest Metazoa 

 it is probable that all the cells lining the digestive cavity 

 are alike and that they all secrete the same digestive fluids; 

 in more complex animals the cells differ in structure in 

 different portions of the tract. Some of these cells form 

 diverticula or blind tubes opening out from the canal ; thus 

 digestive glands are formed which pour particular diges- 

 tive secretions into the alimentary canal. Those most gen- 

 erally present are the salivary glands, opening into the 

 fore-gut, and the liver and pancreas (or where both are 

 united, as often happens among the invertebrates, the 

 hepato-pancreas), which open into the mid-gut. 



Movements of Food in Alimentary Canal. In all of the 

 lower invertebrates except the round worms the food is 

 moved about in the alimentary tract by means of cilia 

 or by general contractions of the body. In all higher 

 forms the contraction of muscle fibres surrounding the 

 canal plays an important part in this movement, though 

 cilia may also be present. In the chordates both longi- 

 tudinal and circular muscles surround the canal and by 

 their rhythmical contractions produce a wave-like con- 

 striction of the canal (^peristalsis^ ^ which passes along the 

 canal from mouth to anus. 



5. Respiratory System. 



Respiration consists in the exchange of gases between 

 the body and the medium which surrounds it. The gas 

 given off from the body is principally carbon-dioxide, one 

 of the products of combustion within the body, while that 

 which must be supplied to it is oxygen. Since oxidation 

 is the one essential feature of destructive metabolism 



