266 GENERAL BIOLOGY 



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

 ter 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 the better when it is remembered that 

 all unpaired parts of the animal, such as 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 th-e 

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

 surface various openings can readily be seen. These are principally, 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 on 

 the fourteenth ; the excretory organs, called nephridia, have two open- 

 ings 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 there is little if any progress 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 



Probably the earthworm illustrates a coelom (Fig. 162) as well as 

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

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

 ance of a tube within a tube, the central one being 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 being remembered that a 



