METAMERISM. 45 



aspects, have been differently used and exposed to different con- 

 ditions of environment. And on the other hand the organism is 

 bilaterally symmetrical, because the two sides have been similarly 

 used and have been exposed to like conditions of environment. 



Metamerism. Another general feature of the earthworm is 

 of great importance in view of the conditions existing in other 

 animals, including the higher forms. The body is marked off 

 by transverse grooves into a series of similar parts like the joints 

 of a bamboo fishing-rod, or like the joints of fingers (Fig. 21). 

 These parts are called metameres, or more often somites, and 

 the body is consequently said to have a meta/meric structure, or 

 to exhibit metamerism. From the outside, the somites appear to 

 be produced simply by regular folds in the skin, like the 

 wrinkles between the joints of our fingers. But as the wrinkles 

 of the fingers are only the external expression of a more funda- 

 mental jointed structure within, so the external folds separating 

 the somites, represent an internal division into successive parts, 

 which affects all the organs of the body, and is a result of some 

 of the most important phenomena of development. 



The explanation of metamerism or " serial symmetry"' 1 is one of the 

 most difficult problems of morphology. But it will be seen farther on that 

 metamerism, so clearly and simply expressed in the earthworm, can be 

 traced upward in ever-increasing complexity to the highest forms of life, 

 and suggests some of the most interesting and fundamental problems with 

 which biology and especially morphology has to deal. Indeed, the 

 comparative study of the anatomy of most higher animals consists very 

 largely in tracing out the manifold transformations of their complicated 

 somites, which under many disguises can be recognized as fundamentally 

 like the simpler somites of the earthworm. 



Modifications of the Somites. The somites differ considerably 

 in different parts of the body. The extreme anterior end is 

 formed by a smoothly-rounded knob called the prostomium^ 

 which is shown by its mode of development not to be a true 

 somite. It forms a kind of overhanging upper lip to the mouth, 

 which lies just behind it on the ventral aspect. Behind the 

 mouth is the first somite, in the form of a ring,* interrupted 

 above by a backward prolongation of the prostomium. 



* In numbering the somites the prostomium must never be reckoned, the 

 first somite being behind the mouth. 



