THE LEECHES OF MINNESOTA 9 



and thus indirectly hastening excretion. The following outlines, Fig. 1, 

 represent some of the more striking shapes assumd by one of the 

 Glossiphonidae. 



UfllUlM 



Fig. i 



These striking changes of form have not been observd among the 

 more elongated, ribbon-like leeches. In these the change of form is 

 practically limited to extension or elongation accompanied with a de- 

 crease in width and to a contraction or shortening in length accompanied 

 with an increase in width. 



Another characteristic external feature is a more or less evi- 

 dent annulation of the body. Nearly all leeches appear to be made 

 up of a series of rings technically cald annuli. Such annulation or 

 segmentation of the body is quite characteristic of a large group of 

 worms cald annulata. This group includes a large variety of 

 marine worms, some freshwater worms, the earthworms and the 

 leeches. 



A careful examination of our common earthworm, also cald 

 angleworm, will disclose the fact that the internal organization is 

 also segmented and that this internal segmentation corresponds to 

 the external annulation. In other words, the annulation seen on 

 the surface of the body represents a condition of the entire organ- 

 ization. For example, a ring in 'the middle region of the body is 

 separated from the ring in front of it and from the ring behind it 

 by a thin transverse partition at each end. Between these parti- 

 tions, that is within the ring, there is a nerve centre and certain 

 nerve fibers, a pair of excretory organs, muscles and so on. The 

 same is true of the ring in front of this and the ring behind it. 

 Indeed all the annuli, excepting a few modified ones of the anterior 

 end and a few modified ones of the posterior end, are quite alike in 

 their make-up. The earthworm, therefore, may be thought of as 

 an animal made up of a number of similar rings or segments joind 

 together end to end in a single row. Each ring or segment in such 



