EARTHWORMS, LEECHES, ETC. 99 



that they cover the surface with earth at the rate of about an 

 inch in five or six years. Besides being an important factor in 

 increasing the fertility of the soil the earthworms furnish a 

 considerable part of the food supply of many birds. Nor 

 should we fail to mention the important part that they play 

 in the welfare of mankind, or rather "boykind," when, dan- 

 gling from a hook, they tempt the hungry fish from its haunts 

 in the shade of the rocks or among the gnarled roots of the old 

 tree. Usually they remain in the ground during the day, 

 wandering about only at night, but sometimes, particularly 

 after heavy rains, they may be found in considerable numbers 

 crawling over the ground in the early morning. As they are 

 often found on hard pavements where they have crawled or 

 been washed by the water many persons think that they have 

 "rained down." 



External Structure. The body of the earthworm is long, 

 cylindrical, bluntly pointed at the anterior end and rounded 

 and flattened at the posterior end. The four double rows of 

 stiff bristles or setae are hardly visible to the naked eye but 

 they may be detected by drawing the worm backward across 

 the hand. Over the mouth is a small lobe called the prosto- 

 mium, and a short distance behind it is a broad thickened ring 

 or girdle, the clitellum. This is a glandular structure which 

 secretes the cases in which the eggs are laid. 



Internal Structure. If a careful incision is made along the 

 dorsal line extending from behind the clitelleum to the anterior 

 end of the body the sides of the body-wall may be fastened 

 aside so as to expose the internal organs. The body-wall is 

 made up of a thin transparent covering, the cuticle, just be- 

 neath which is a less transparent layer, the epidermis, and two 

 layers of muscles, an outer circular layer and an inner longi- 

 tudinal layer. The space between the body-wall and the 

 alimentary canal is the body-cavity, or ccdom. It is divided 

 into sections or segments by thin membranes, the septa. The 

 body of the earthworm may be compared to two tubes, a 

 larger outer one represented by the body-wall and a smaller 

 inner one, the alimentary canal, extending from one end 

 to the other. The space between these is the body-cavity. 



