STRUCTURE OF THE EARTHWORM 135 



crawling surface or belly and the opposite more rounded sur- 

 face or back, and the phenomenon is called dorso-ventral dif- 

 ferentiation, which becomes more plainly marked in higher 

 types of animals. 



Bilateral Symmetry. All of the organs of the body which do 

 not lie on the median line are found in pairs, one on each side of 

 a plane passing through the longitudinal center of the body. 

 The mouth, anus, and entire digestive tract are unpaired and 

 lie in the median plane; so do the main blood-vessels, but all of 

 the reproductive organs, excretory organs, nervous system, mus- 

 culature, setae, etc., are paired structures, so that one entire 

 side of the earthworm is an exact replica of the other. This 

 phenomenon, also characteristic of the higher animals, is called 

 bilateral symmetry. 



External Apertures. Some of these are too minute to be seen, 

 but others can be easily made out. Two pairs of minute pores 

 (openings of the seminal receptacles) are on the ventral surface 

 between the gth and loth and zoth and nth somites; a pair of 

 male genital openings are on the i5th and a pair of female 

 genital openings are on the i4th. On the ventral surface also 

 there are two extremely minute openings of the excretory 

 organs (nephridia) in each somite, except the first three or 

 four and the last. With the exception of the anus, all of the 

 openings posterior to the male genital pore are too minute to 

 be seen. 



While most of the external openings are on the ventral sur- 

 face, some are on the dorsal surface. Here, for example, are the 

 dorsal excretory pores, one to each somite, after the loth, in 

 the annular creases, and very difficult to see. 



Setae. There are no true appendages on the worm's body, 

 but if the animal is drawn gently through the fingers, fine bris- 

 tle-like structures may be felt. These are setae or bristles, 

 easily seen with a hand lens. There are eight setae to each 

 somite, -arranged in four double rows on the ventral surface and 

 the sides. They aid the worm in locomotion by catching into 

 the earth which acts as a fulcrum. The flattened tail of some 

 forms (Lumbricus terrestris) also serves a useful purpose in 



