EARTHWORM 87 



nature of the soil. In all cases, the burrows are formed by the 

 Earthworm literally 'eating a hole' in the soil. All the materials 

 taken from the burrow pass into the alimentary canal through the 

 mouth opening, then through the entire length of the canal, and 

 finally are egested through the anal opening as castings, or feces. 

 The castings are voided at the surface around the entrance to the 

 burrow, where they form tiny mounds of earth. The great number 

 of Earthworms present in the soil of many regions, together with 

 their habit of almost continual burrowing, makes it possible for 

 them to bring considerable quantities of soil from lower depths to 

 the surface. It has been estimated that in some localities as much 

 as 18 tons of soil per acre are annually brought to the surface in this 

 manner. 



This method of burrowing by eating through the soil not only 

 provides the Earthworm with a method of home-building, but it 

 also constitutes the chief source of its food supply, for, as the 

 soil passes through the alimentary canal, much of the organic 

 material there present is utilized for food. In addition to obtaining 

 food from the materials in the soil, the Earthworm when it comes 

 to the surface, generally at night, secures various other materials 

 which can be utilized for food. 



In the ingestion of food material the pharynx plays an impor- 

 tant part. When the mouth is applied to some suitable object, 

 the muscle fibers, which run from the pharynx to the body wall, 

 contract. This enlarges the pharynx, and the resulting suction 

 aids in drawing the food through the mouth and into the pharynx. 

 The ingested material passes through the pharynx and esophagus. 

 In the latter, a secretion from the calciferous glands, consisting 

 largely of calcium carbonate, is added to the food, which probably 

 aids in neutralizing any excess acidity which may be present. The 

 material passes next into the crop, which serves only as a temporary 

 storage place, and then into the muscular gizzard, where it is 

 ground. Finally, the ingested material is passed into the tubular 

 intestine and as it moves slowly through this part of the alimentary 

 canal, the cells of the lining epithelium carry on the digestive and 

 absorptive processes as noted just above. 



2. Vascular System 



In the Earthworm there is a closed vascular system which has 

 as its function the transportation of various kinds of materials 



