LUMBRICUS. 199 
Like all other Aznelida, the earthworm is plano-sym- 
metric, though the absence of appendages makes this less 
evident than in other classes. 
The body is constricted throughout into a series of 
about 150 segments, but there are no annuli. The segments, 
from about 29 to 35,* have a swollen appearance 
mectibee anda yellowish colour. They form the cdcted/um. 
‘ The mouth is at the anterior end, overhung by 
a prostomium and bordered by the Zertstomium. At the 
extreme posterior end is the anzs. 
As in the leech, there are neither gills nor appendages. 
If the body of the worm be drawn through the fingers from 
tail to head it will rasp with some resistance to the fingers. 
This is due to the presence of minute se¢e which are found 
oneach segment. The setee are in pairs and are arranged in 
two ventral and two lateral rows. Each segment, therefore, 
has eight sete. They naturally project backwards and aid 
the locomotion of the worm in the same way as the appen- 
dages of Avenicola. In the mid-dorsal line is a row of median 
dorsal pores, occurring between each segment from about 
the gth backwards and communicating with the celom. A 
pair of minute nephridiopores open on the ventral surface 
of each segment (except the first two), but they are too 
small to be recognised without the aid of a lens. On the 
15th segment there is a pair of ventral openings with tumid 
lips, the male genzfa/ openings, and on the segment (14th) 
in front are the two female genital apertures.. Between seg- 
ments 9, 10 and 11 there are the two paired openings of 
the spermathece. 
The body is covered by a cuticle with simple ectoderm, 
forming a flexible but firm envelope. Scattered 
throughout the ectoderm are numerous uni- 
cellular glands, specially abundant in the region 
of the clitellum. 
There are no eyes nor otocysts, but the prostomium has 
sense-organs for perception of contact and per- 
haps of taste. The alimentary canal is exposed 
by making a median dorsal incision along the body of the 
worm. ‘The mouth passes into the muscular pharynx, from 
External 
Integu- 
mentary. 
Sensory. 
* There is great variation in the position of the clitellum. 
