104 THE LEECHES OF MINNESOTA 



of this region are of simpler structure than the typical complete ones 

 adjoining. Clitellar glands are greatly developed and form a thick 

 layer just within the longitudinal muscle layer and extending from the 

 clitellum nearly to the anus. They are arranged in four longitudinal 

 bands on each side leaving narrow neural, median dorsal, and lateral 

 spaces clear. The latter are occupied by the lateral vessels which ex- 

 hibit metameric enlargements in the somites of the posterior region. 



Ten pairs of large nephridiopores are present on the latero-ventral 

 region of somites XIV to XXIII inclusive. They lie in annulus c6. 

 No especially metameric sensillae have been certainly distinguished but 

 numerous small sense organs arranged in transverse rows in many 

 of the annuli are present. 



Owing perhaps to the different methods by which my material 

 has been prepared the annulation varies in a manner which, combined 

 with its complexity, is very confusing, and a complete analysis has 

 not been reached. Figure 21 Plate III exhibits a case which approxi- 

 mates the most frequently occurring condition, together with the in- 

 terpretation of somite limits which has been based upon a study of 

 the annuli themselves, the nephridiopores, nerve ganglia and partially 

 of the peripheral nerves. 



Complete somites have the full number of tertiary annuli (ci to 

 C12) developed and in many one or two of these, usually in the 

 cephalic third of the somite, are divided into two, making in the latter 

 case fourteen annuli ; but it is in connection with this feature and the 

 simpler somites at the ends of the body that the variability occurs. 



Unlike Actinobdella the mouth is situated far back in the oral 

 sucker at III/IV or possibly within the limits of IV. The rather short 

 proboscis ends in VIII where it receives the several ducts of the 

 diffuse salivary glands occupying the pre-clitellial region. The 

 stomach is moniliform, constricted into six spheroid chambers occupy- 

 ing somites XIV to XIX inclusive and entirely without lateral caeca. 

 The last one passes into a long capacious unpaired caecum which 

 shows no apparent traces of its dual origin and extends with slight 

 sacculations to a point immediately beneath the anus. The stomach 

 and caecum, as might be expected, have a precisely similar histological 

 structure and both have a green color owing to the presence of numer- 

 ous branched pigment cells in their walls. The intestine arises from 

 tbe dorsum of the last gastric chamber in XIX by a constricted open- 

 ing and lies dorsad of the caecum throughout its length. At its com- 

 mencement it bears a pair of short wide pouches which project for- 



