238 Elementary Biology. 



tions of the intestinal walls over and above the typhlosole 

 fold must also serve to greatly increase the area of absorp- 

 tion, an end of great importance when we bear in mind the 

 relatively small quantity of food matter which must be pre- 

 sent in a large amount of humus swallowed. The intestine 

 is almost enveloped in a yellowish granular tissue, which is 

 probably of use in rendering % the food capable of absorption. ' 

 The intestine terminates at the anus, the external opening 

 which we have already noted in the examination of the ex- 

 ternal characters. 



When we examine microscopically the constituent tissues 

 of the alimentary canal we find them to consist of a layer 

 of columnar cells internally, surrounded by connective 

 tissue, composed of delicate fibres and branched cells, in 

 which are embedded numerous blood-vessels followed by an 

 inner layer of circular, and an outer layer of longitudinal 

 muscular fibres. 



Nutrition. The physiology of nutrition is extremely 

 simple, for the worm, living as it does on decaying vegetable 

 matter, takes into its alimentary canal food already partially 

 digested, or at least rendered very easily digestible by the 

 natural processes of decomposition taking place in the en 

 vironment. No doubt the salivary and other secretions 

 which the woim mixes with the food accelerates this process 

 or has a similar decomposing effect on the vegetable matter 

 which has not already undergone disintegration. The 

 nitrogenous and other compounds are absorbed by the cells 

 lining the alimentary canal, although the part played by the 

 yellow cells, which, as has been already mentioned, line the 

 outer surface of intestine, is still looked upon by some as 

 doubtful. The excreta, consisting of the earth from which 

 the food matters have been extracted, are ejected from the 

 anus and accumulate often in large piles at the mouth of 

 the worm's burrow. The nutritive substances having been 



1 By some biologists this tissue is considered as vasifactive, or blood - 

 producing. 



