12 THE LEECHES OF MINNESOTA 



placed in two groups, one having three primary annuli and one five 

 primary annuli in a typical somite. 



The differences in the number of annuli and the modifications 

 of them in the anterior and posterior somites of the leeches of 

 Minnesota are clearly and fully presented in the systematic portion 

 of this report. It may be well, however, to note that in all leeches 

 several of the posterior somites (six or seven) are fusd to form the 

 attaching sucker and the number of annuli is more or less reduced 

 in the anterior somites. 



More precise and accurate descriptions of the various species 

 and a more satisfactory identification of each species are made pos- 

 sible by numbering the somites and annuli of each somite from the 

 anterior end back. The somites are now generally designated with 

 the Roman numerals and the annuli with the Arabic. A few ex- 

 amples will make this clearer than can a detaild description. A 

 structure or marking on the first annulus of the twelfth somite 

 would be located thus, — XII al; something on the boundary line 

 between the first annulus and the second of the same somite, thus, 

 XII al/a2; and something between the twelfth and the thirteenth, 

 thus, XII/XIII. To indicate the relation of the secondary to the 

 primary annuli the letters a, b, c, &c are used, a indicating a primary 

 annulus; b, a secondary annulus and c, a tertiary annulus. The fol- 

 lowing diagram, Fig. 3, illustrates the derivation of a typical somite 

 of five annuli and a typical somite of six annuli from one of three 

 annuli. 



Three annuli to a typi- Five annuli to a typi- Six annuli to a typical 



cal somite as in Glos- cal somite as in Ma- somite as in Actinob- 



siphonia. crobdella. della. 



Fig. 3. 



The following diagram, Fig. 4, illustrates how annuli become 

 divided into two, four or six annuli and how a somite of four or 

 twelve or fourteen annuli may be derivd from a somite of three 

 annuli. 



