METAMERISM. 121 



metameric structure, or to exhibit metamerism. From the out- 

 side, 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 expres- 

 sion of a more fundamental jointed structure within, so the exter- 

 nal 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 de- 

 velopment. 



The explanation of metamerism, or ''serial symmetry," is not so easy as 

 that of bilateral symmetry; it is in fact 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-increas- 

 ing 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 trans- 

 formations 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. 



The somites from the 1st to the 27th are rather broad, and 

 gradually increase in size. Between the 7rh and 19th somites 

 are a variable number, which are often swollen on the ventral 

 side, forming there ylandular prominences by which the worms 

 adhere to each other during copulation. Between the 2Sth and 

 35th (the number and position varying slightly in different speci- 

 mens) the somites are swollen above and on the sides, and the 

 folds between them are scarcely defined except on the ventral 

 aspect. Taken together, they form a broad, conspicuous girdle 

 called the diteUum (Fig. 72, c), whose function is to secrete the 



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

 first somite being behind the mouth. 



