The Earthworm 267 



end. On the ventral surface are also found the mouth, anus, reproductive, 

 and excretory openings, as well as peculiar bristle-like setae. These 

 latter will be discussed under locomotion. 



The earthworm, like the frog, is bilaterally symmetrical. A median 

 dorso-ventral line drawn through the worm divides it into two equal 

 parts. This will be understood better when it is remembered that all 

 unpaired parts of the animal, such as the mouth, anus, central blood 

 vessel, etc., would be cut into two equal parts by a medial section, while 

 all paired portions such as setae and reproductive openings would have 

 one-half of such paired portion on each side of the animal. 



The dorsal excretory pores, one to each somite posterior to the 

 tenth, lie in the constrictions and are difficult to find, but on the ventral 

 surface various openings can readily be seen. Principally, these are two 

 pairs of minute pores between the ninth and tenth and the tenth and 

 eleventh somites, coming from the seminal receptacles. The male genital 

 openings are on the fifteenth, and the pair of female genital openings are 

 on the fourteenth somites. The excretory organs, called nephridia, have 

 two openings on each somite behind the first three or four and anterior 

 to the last. Practically all of the ventral openings posterior to the male 

 genital pore, with the exception of the anus, are too small to be seen 

 with the unaided eye. 



The animal moves along primarily by alternate rhythmic constric- 

 tions of the longitudinal and circular muscles of the body-wall which 

 contract and elongate successive regions of the body. There are eight 

 chitinous setae to each somite, easily felt if the animal be drawn between 

 the fingers. An ordinary hand-lens will show them quite clearly. There 

 is then a double way in which the worm moves, the muscular action 

 furnishing the contraction and expansion and the setae furnishing cog- 

 like projections by which the worm can make forward progress. This 

 is well exemplified by the fact that if an earthworm be placed on a 

 highly polished surface, little if any progress is made by it. 



Muscles are attached to the inner parts of the setae, making it pos- 

 sible to shift their positions. The flattened tail of Lumbricus terrestris, 

 serves as an anchor, while the anterior portion of the animal's body lies 

 on the surface of the earth. 



INTERNAL STRUCTURE 



The earthworm illustrates a coelom (Fig. 162) as well probably as 

 any form which could be given the student, for upon making either 

 dorsal or ventral longitudinal incision the animal will give the appear- 

 ance of a tube within a tube. The central tube is the digestive tract" 

 held in its central position by little thin membranes, or walls, running 

 from each outer constriction. These walls are called septa ( ) 



or dissepiments ( ). There are here, then, many 



coelomic cavities which can be clearly seen. It will be remembered 



