82 MANUAL OF ANIMAL BIOLOGY 



A. External Structure 



The Earthworm has an elongated, cylindrical, segmented body 

 which may vary in length from a few inches to a foot or more. 

 Rare species of Earthworms are known which may even reach a 

 length of several feet. One end of the body tapers down to a some- 

 what blunt point in contrast to the opposite end, which is rather 

 flattened. A study of the animal shows that the more pointed end 

 is the anterior, or head, end, and the opposite is the posterior, or 

 tail, end. The Earthworm may be said, therefore, to show 

 antero-posterior DIFFERENTIATION. Also, the worm, when 

 crawling along, maintains as a general rule the same part of the 

 body in contact with the solid surface. An examination of the 

 body will show that this lower, or ventral surface, on which the 

 worm crawls, is somewhat flattened as compared with the upper, 

 or dorsal surface. A differentiation between these two sur- 

 faces, as shown in the Earthworm, is known as dorso-ventral 



DIFFERENTIATION. 



The number of segments present in the body varies considerably 

 in individual worms, but all the segments have essentially the 

 same structure. Certain minor segmental variations may be noted. 

 The first segment at the anterior end of the body has an over- 

 hanging dorsal projection, known as the prostomium, which forms 

 a lip-like structure over the mouth. In the last segment of the body 

 is the slit-shaped anal opening. About one-fourth of the distance 

 back from the anterior end a few segments are differentiated to 

 form an enlarged glandular structure, termed the clitellum, 

 which functions in connection with reproduction. (W. fs. 59 ; 60.) 



If an Earthworm is drawn through the fingers a roughness will 

 be noted. This is due to the presence in each segment of tiny 

 bristles, or setae, which project through fine openings in the body 

 wall. Each segment has eight setae arranged in four pairs. The 

 setae are in corresponding positions in each segment, so that four 

 double rows of setae are formed which run the entire length of the 

 body. Two of the double rows are on the ventral surface : one 

 double row is on the right, and one on the left side. 



In a transverse section of the body, the finer structure of the 

 setae may be observed. Each consists of a bristle embedded in 

 the body wall, long enough to project a slight distance beyond the 

 exterior surface of the body at one end and, at the other end, to 



