CASTLE: NORTH AMERICAN RHYNCHOBDELLID^E. 57 



The color pattern is somewhat similar to that of plana, but the contrasts are 

 less striking and the colors less brilliant. The general color effect of the dorsal 

 surface is a grayish brown. Marginal spots of light yellow are present, as in 

 plana, on the non-sensory rings, but they are smaller and do not extend so far 

 mesiad from the margin of the body. Practically all the larger papillae appear 

 as small white spots iu a generally dark background. 



The median vitta is not a continuous light band as in plana, but is inter- 

 rupted at regular intervals by spots of a darker color than the general dorsal 

 surface. It begins as a narrow median light band on the head and neck, con- 

 stricted or sometimes interrupted in the posterior part of somite vi., less often 

 constricted or interrupted in somite v. also. About in annulus 19, somite ix., 

 begins a narrow dark band which continues to the middle of somite xii. 

 Then come alternating light and dark spots, three of each. A light spot ex- 

 tends over four annuli, a black spot over five as follows : Light spots, annuli 

 29-32 (Figure 6), 38-41, 47-50; dark spots, annuli 33-37, 42-46, 51-55. An- 

 other light spot covers rings 56-64 or 65, broadening out posteriorly so as to 

 include the paired papillae of somites xxiii. and xxiv. (Figure C). This is fol- 

 lowed by a median dark spot extending back past the anus to the margin of 

 the posterior sucker. 



The posterior sucker is marked by alternating light and dark rays, very 

 much as in plana (Figure 6) ; it also bears papillae like those of the body 

 farther forward. 



Ventrally the body is light gray in color, owing to the presence there of 

 scattered pigment flecks, which, however, are not arranged in longitudinal 

 bands as in plana. 



V. MUTUAL RELATIONSHIPS OF THE SPECIES 

 DESCRIBED. 



The species described in this paper, with the exception of heteroclita, 

 fall naturally into two distinct groups (Figure D, page 58), which may 

 be designated respectively the stagnalis and the parasitica groups. The 

 former includes the three species stagnalis, elongata, and fnsca ; the 

 latter, parasitica and elegans, with the closely related European species, 

 complanata and concolor. Heteroclita occupies a somewhat isolated 

 position intermediate between these two groups. 



As arranged in Figure D., the species form a series in which there is 

 from left to right an increasing degree of complexity of structure. This 

 appears from an examination of rugosity, somite structure, crop diverti- 

 cula, and certain other characters. 



In the species of the stagnalis group (1) there is a single pair of eyes 

 derived from the sensillee of somite iii., (2) the genital pores are separated 

 by a single ring, namely, the middle (sensory) ring of somite xii., and 



