80 THE WORM GROUP 



some one has produced life from chemicals, but it is not 

 believed at the present time to be possible. 



67. The Earthworm is the simplest and best animal to 

 illustrate the annelid group of true worms. 



When one examines a living earthworm, the head end 

 can be determined as the one which first moves forward. 

 Actually there is no head nor are there special sense 

 organs. The muscles in the front end are stronger and 

 the body rounder than in the back end. The back, or 

 dorsal (dor's'l) part, of the worm is exposed to the light 

 and is darker in color than the rest. This surface is 

 rounder than the opposite (under) one which is in con- 

 stant touch with the dirt when the worm is crawling. 

 The flat surface upon which the worm crawls is the ven- 

 tral (ven'tral) surface. 



The body of the earthworm is made up of a number of 

 segments (rings) which are marked off by shallow grooves. 

 Some of the segments in the front end are larger than 

 those that make up the back end, but all are similar in 

 shape. The number of segments depends mostly upon 

 the age of the earthworm, and is from 60 to 150 in full- 

 grown worms. 



68. Locomotion. — The earthworm crawls by means of 

 short, stiff bristles used as legs, the seta? (se'te : Latin, 

 seta, bristle), which are found in all of the segments 

 except the first two or three. These setce are arranged 

 in four rows, two in each row. To understand how the 

 setse are used in the locomotion of the earthworm it is 

 necessary to know that the body wall contains two mus- 

 cular layers. In the outer layer the muscles running 

 around the body are called circular muscles. The inner 

 layer, consisting of a number of bands running in the 

 direction of the length of the body, are called longitudi- 

 nal muscles. The contraction of the circular muscles 



