LOCOMOTION 
179 
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 which is exposed to the light is darker in 
color than the rest. This surface is rounder than the opposite 
(under) one which is in constant touch with the dirt when 
the worm is crawling. The flat surface upon which the 
worm crawls is the ventral (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. It is from 60 to 150 in full-grown worms. 
162. Locomotion. — The earthworm crawls by means of 
short, stiff bristles used as legs, the setce (se'te: Latin, seta , 
bristle), which are found in all the segments except the 
first two or three. These setae are arranged in four rows, 
two in each row. To understand how the setae are used in 
the locomotion of the earthworm it is necessary to know 
that the body wall contains two muscular 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 longitudinal muscles. The contraction of the 
circular muscles lengthens the body and the contraction of 
the longitudinal muscles shortens it. The setae are con¬ 
nected with the longitudinal muscles. By pointing the 
setae backward and bracing them against the ground, the 
worm can push itself forward. By pointing the setae forward 
the worm can instantly change the direction of its movement. 
This is the reason why it is so difficult for a robin to pull 
an earthworm from its burrow. Often the robin will tear 
the worm apart, so firmly do these setae hold. 
