132 ORGANS AND ORGAN SYSTEMS 



the crawling surface or belly and the opposite more rounded 

 surface 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 

 (seminal receptacles) are on the ventral surface in the eighth 

 and ninth somites; a pair of male genital openings are on the 

 fifteenth and a pair of female genital openings are on the four- 

 teenth. On the ventral surface also there are two extremely min- 

 ute 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, 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 bristle- 

 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 anchoring 



