198 



ANIMALS 



digestive cavity, into which the digestive enzymes are se- 

 creted, in which digestion takes place and from which the 

 soluble products are then absorbed. The long digestive tube 

 gives a large absorbing surface, but in some cases, as in the 

 earthworm, the surface is further increased by a longitudinal 

 fold which hangs from the dorsal side of the canal and gives the 

 lumen of the canal a crescentic form in cross section. The 



Fig. 106, 



FiG. 105. Fig. 107. 



I'TG. 105. — Cross section of the intestine of nereis showing the glandular 

 epithelium and blood capillaries (black). 



Fig. 106. — A part of the preceding figure enlarged. The upper two-thirds 

 of the figure is the epithelium. Below that is a blood capillary. Then follows 

 a layer of longitudinal muscle fibres cut across and a layer of circular muscle 

 fibres lying in the plane of the section. The lower layer is an extremely thin 

 epithelium lining the outer surface of the intestine. 



Fig. 107. — A surface view of the inner surface of the intestinal epithelium. 

 The cells are outlined by a network of supporting fibres. 



salivary glands are a simple type of a compound gland. The 

 glandular epithelium is pushed outward into the body cavity 

 and is greatly folded so that a large glandular surface occupies 

 a small space. The part by which the gland is connected with 

 the intestine forms a duct through which the secretion is poured 

 into the digestive cavity. In nereis the greater part of digestion 

 is doubtless due to the activity of the unicellular glands. 

 457. The glandular intestinal epithelum is only a lining of a 



