AN EARTHWORM 55 



character in a segmented animal and is in sharp contrast to the 

 condition of the somites in most arthropods. 



Notice the moist, sHmy surface. Moisture is necessary to the 

 animal's existence ; this accounts largely for its nocturnal and 

 subterraneous habits. Notice also the red blood vessels through 

 the semitransparent body wall. What movement of the blood can 

 you detect ? What are the differences between the dorsal and the 

 ventral surfaces ? Notice the difference between the anterior and 

 the posterior ends. The forward end is the older ; the animal 

 grows in length by adding new somites to the hinder end. Notice 

 the ventral position of the mouth and the terminal position of the 

 anus ; note also the thickened ring around the body not far from 

 the forward end. This is the clitellum ; its function will be ex- 

 plained in speaking of the reproductive organs. The animal is 

 without organs of special sense ; numerous minute, tactile sense 

 organs which are sensitive to light and other stimuli are, however, 

 present. These are distributed along the body but are especially 

 abundant toward its anterior end. Note the greater sensitiveness 

 of this part of the body. 



The head of the animal may be divided into two portions : the 

 prostomium, the median dorsal projection overhanging the mouth, 

 and the indistinct peristomium, which contains the mouth and is 

 marked off from the prostomium by fine, transverse lines. What 

 somites are included in the clitellum? On the ventral surface of 

 the fifteenth somite a pair of prominent, transverse slits will be 

 seen. They are the external openings of the sperm ducts. On 

 the fourteenth somite look with the hand lens for the two minute 

 openings of the oviducts ; they are difficult or impossible to see 

 except during the reproductive period of the animal. Other 

 external openings in the ventral surface which are usually too 

 small to see are the following : the two pairs of openings of the 

 sperm receptacles between the ninth and tenth and the tenth and 

 eleventh somites ; and those of the kidney tubules, or nephridia, 

 which open through the body wall to the exterior by minute 

 pores on either the ventral or the lateral side near the anterior 

 border of the somite. Look for them with a hand lens. 



