io THE LEECHES OF MINNESOTA 



an organism is technically cald a somite or metamere, and the 

 condition of being thus built up is cald metamerism. 



The earthworm presents metamerism in a relatively simple 

 form. In the leeches, however, the external annulation does not 

 correspond to the real metamerism. Each true somite or metamere 

 includes several of the externally evident rings or annuli. Conse- 

 quently the number of annuli in the leech is always greater than 

 the number of somites or metameres. 



Moreover, there are no evident partitions between the somites 

 to aid in determining the limits of even a typical somite. But 

 careful study has disclosd the fact that the nerves have definit 

 relations to the annuli and that other internal structures present 

 certain definit relationships, so that we now can determin the 

 limits of the leech somite quite as definitly as we can those of an 

 earthworm. 



It is now generally accepted that the leech body is composd 

 of thirty three or thirty four somites. The number of annuli varies 

 considerably in the different species but the number of somites is 

 always the same. This is one of the features in which the leeches 

 differ from the other groups of annulata. 



The limits of the somite adopted in this report are not those 

 current prior to 1900. It is not necessary to give more than a brief 

 discussion of the limits of the leech somite in this report since any 

 one interested in the question can find a clear and full presentation 

 of it in Castle's paper on "The Metamerism of the Hirudinea'' in 

 Vol. XXV, 1900, of the Proceedings of the American Academy of 

 Arts and Sciences. This paper was reprinted as No. 108 of the 

 Contributions from the Zoological Laboratory of the Museum of 

 Comparative Zoology at Harvard College. Also in Moore's paper 

 entitled "A Description of Microbdella biannulata with Especial 

 Regard to the Constitution of the Leech Somite", which appeard 

 in the Proceedings of the Academy of Natural Sciences of Phila- 

 delphia of 1900. 



The earlier writers recognizd only the annuli, which they num- 

 berd consecutively from the anterior end back. They located 

 structures by direct reference to the number of the annulus. In 

 1862 Gratiolet pointed out that the annuli of the medicinal leech 

 are not all alike but that they are arrangd in similar groups 

 (somites) within which are always found certain structures. Later 

 it was generally admitted that the ganglia (groups of nerve cells 



