132 ORGANS AND ORGAN SYSTEMS 



A. OCCURRENCE, HABITS AND MODE OF LIFE OF EARTH- 

 WORMS. Earthworms are widely distributed over the earth, and 

 six or seven different species are known, some growing to giant 

 size (three to four feet long). Closely allied forms are partly 

 earth-dwelling, partly water-dwelling forms, and some live 

 entirely in water, while one type (Dendrobaena) burrows in 

 the green ice of glaciers as an earthworm burrows in the earth. 



Earthworms live in winding burrows formed by eating their 

 way through the earth, the burrows running through the soil 

 at a depth of from five to six inches to several feet. The worms 

 are nocturnal for the most part, coming out of their burrows at 

 night to forage. During the day they lie in their burrows, 

 mouth end up and close to the surface; at night they emerge, but 

 usually remain anchored by their tails, exploring the region of 

 their burrows throughout the area covered by their body 

 radius. Pebbles, dirt, leaves and other small objects lying 

 within this radius are swallowed or dragged into the burrows, 

 where the small stones are used to line the walls and to cover 

 the opening in the daytime. The dirt that is swallowed with 

 nutritious matter, such as leaves, animal remains, etc., is slowly 

 passed through the digestive tract; the nutritious parts are 

 digested out, while the residue, consisting mainly of dirt, is 

 voided to the outside through an opening at the opposite end, 

 the anus. This defecated material is deposited on the outside 

 of the burrow, where as small mounds, or " castings" or ''fasces, ' 

 they are familiar to every observer. Darwin, who made a 

 special study of earthworms, has shown that enormous masses 

 of earth pass in this way through the bodies of worms, as much 

 as eighteen tons per acre per year in regions where earthworms 

 abound. He has also shown that the entire surface of a field 

 with great rocks upon it, in the course of several years, will 

 be buried by the castings of worms, while walls and even build- 

 ings are similarly sunk into the earth in the course of time. 



As a creeping, crawling, and burrowing thing, the earthworm 

 is well adapted to its mode of life. Its long flexible body makes 

 it particularly adapted to its burrowing habits, and a thought- 

 ful student will puzzle over the problem whether its elongated 



