THE LEECHES OF MINNESOTA 



ii 



and nerve fibers) are definit internal criteria of the somites. Now 

 in a leech having three annuli to a typical somite each ganglion 

 gives off three nerves on each side to certain definit parts of the 

 somite. Prior to 1900 the annulus carrying the dorsal sense organs 

 of such a leech was considerd the first annulus of the somite. This 

 annulus also containd the ganglion. The nerves coming from this 

 ganglion, however, were distributed to the annuli of two somites. 



Castle concluded from his observations that if the ganglion is 

 an important element of a somite it is only natural to expect all of 

 its nerves to be supplied to that somite, and accordingly he decided 

 that the anterior or first nerve went to the first or anterior annulus, 

 the second nerve to the second annulus, which contains the gang- 

 lion and bears the dorsal sense organs, and the third nerve to the 

 third or posterior annulus. These limits also brought other inter- 

 nal structures into more satisfactory relationships and presented 

 the somite in all details of structure as a complete unit in itself. 



Moore had reacht the same conclusion independently. 



In the accompanying diagram, 

 Fig. 2, the somite as determind 

 by Castle and Moore, and adopted 

 in this report, is indicated on the 

 right side and the limits according 

 to the older writers are indicated !2\2l 

 on the left. 



Not only does the number of 

 annuli vary in the different species, 

 while the number of somites is 

 constant, as has already been 

 stated, but the number of annuli 

 in different somites of any given 



species varies. The anterior and posterior somites always have a 

 smaller number of annuli than the somites of the middle region of the 

 body. The typical number of annuli to a somite of any species is de- 

 termind by the number in the somites of the middle region of the 

 body. The annuli of a somite may also be equal or unequal in size. 

 Frequently some annuli are only partially divided, and in some species 

 certain annuli are always divided only on the dorsal or ventral side. 



With regard to the annulation nearly all of the leeches can be 



Fig. 2 



