CHAPTEE VII. 



THE BIOLOGY OF AN ANIMAL. 



THE COMMON EARTHWORM. 

 (Lumbricus agricola, Hoffraeister ; L. terrestris, Linnseus.) 



have now to study the biology of some animal which will 

 introduce us to those structures and functions which are charac- 

 teristic of animals generally, and will afford a basis for comparison 

 with the plant. Any higher animal would serve this purpose, 

 but the common earthworm is a peculiarly favorable object of 

 study, because of the simplicity of its structure, the clearness of 

 its relation to other animals standing above and below it in the 

 scale of organization, and the ease with which it may be procured 

 and dissected. Earthworms, of which there are many species, 

 are found in all parts of the world, extending even to isolated 

 oceanic islands. In the United States there are several species, 

 of which Lumbricns agricola is much the most common. This 

 is the familiar earthworm, which has long been known but was 

 first scientifically described under the name Lumlrrieus terrestris 

 by Linnaeus more than a hundred years ago. It was afterwards 

 found necessary to change the name to L. agricola. 



Mode of Life, etc. Earthworms live in the earth, burrowing 

 through the soil at a depth varying from a few inches to several 

 feet. Here they pass the day-time, crawling out at night or after 

 a shower. The burrows proceed at first straight downwards, and 

 then wind about irregularly, sometimes reaching a depth of six or 

 eight feet. The earthworm is a nocturnal animal, and during the 

 day lies quiet in its burrow near the surface, extended at full 

 length, head uppermost. At night it becomes very active, and, 

 thrusting the fore end of the body far out, explores the vicinity 

 in all directions, still clinging "fast, however, to the mouth of the 

 burrow by the hinder end. In this way the worm is able to for- 

 age, seizing leaves, pebbles, and other small objects, and dragging 



