METAMERISM. 45 



aspects, have l)een differently used and exposed to different con- 

 ditions of environment. And on the other liand the (jri^anisni is 

 hilaterally synnnetrical, because tlie two sides have been similarlv 

 used and have been exposed to like conditions of enviroinneiit. 



Metamerism. Another general feature of tlie eartliwoi'in is 

 of great importance in view of the conditions existing in other 

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

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

 of a band)00 lishing-rod, or like the joints of lingers (Fig. 21). 

 These parts are called inetameres^ or more often somitex, and 

 the body is consequently said to have a Qnetameric structnre, or 

 to exhibit metamerism. From the outside, the somites a})]iear to 

 be ]3i'oduced simply by regular folds in the skin, like the 

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

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

 mental jointed structure within, so the external fohls sei>arating 

 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' is one of the 

 most difficult problems of morphology. But it will be seen fartlier 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 j^rostofniinfi, 

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

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

 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 2:)rolongation of the prostomium. 



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

 first somite being heJiind the mouth. 



