44 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Somite in., within the anterior part of which lies the mouth (or., Figure 20), 

 is ordinarily bianuulate, as are also somites iv. and xxv. (Figures 19, 35, 36). 

 But iu the section shown in Figure 20, ring 3, the anterior annulus of somite 

 III., appears conspicuously subdivided, a rather unusual condition. On account 

 of the obliquity of the section, the tirst three somites appear in that figure a 

 little too long in proportion to their vertical dimensions. The sensilla shown 

 in the anterior portion of ring 3 in Figure 20 is probably not one of the seg- 

 mental sense-organs, for it is found on the wrong half of ring 3. 



Somites v.-xxiv. are triannulate, as in G. fusca. 



Somites xxvi. and xxvii. (reckoned as uniannulate) usually appear divided 

 at the margin only into a broader anterior and a narrower posterior part. 



Compared with the species already described, the somite composition of G. 

 heteroclita is about the same as that of G. fusca, somite abbreviation being less 

 extensive in these species than in stagnalis and elongata. 



Eyes, usually six, the anterior pair small and generally, though not always, 

 close together in ring 5 (Figures 35, 36, Plate 8). Sometimes this pair of 

 eyes lies in ring 6 ; occasionally the pigment of one or both eyes is wanting 

 altogether. 



The second and third pairs of eyes are most often found in rings 7 and 8 

 respectively, but one pair or the other or both may lie a little anterior or a 

 little posterior to the ordinary position (compare Figures 35 and 36). 



The first and second pairs of eyes are directed forward and toward the side ; 

 the third pair is directed backward and toward the side (Figures 20, Plate 5 ; 

 Figures 35, 36, Plate 8). The eyes in this species seem to belong to somites 

 in., IV., and v., respectively (Figure 20) ; but it is possible (though I think 

 hardly probable) that a more careful study of the nerve connections would 

 show that in this species, as in G. elegans (Figure 29, Plate 7), they have been 

 derived from the sensillse of somites ii.-iv. If so, the eyes have undergone a 

 farther displacement backward in this species than in the case of G. elegans 

 (compare Figures 20 and 29). 



Oral Slicker, formed by somites i.-iv. (Figure 20). 



Mouth (or.. Figure 20), in the anterior part of somite in., usually a little an- 

 terior to the first pair of eyes. 



Posterior sucker, as in other species, slightly longer than broad (Figure 19). 



C. TtEPRODUCTIVE OrGANS. 



Male and Jemale genital ducts open between the first and second rings of 

 somite xii. (rings 28 and 29, Figure 19) by a common pore, a condition pecu- 

 liar, I believe, to this species. 



Blanchard ('94) is certainly in error in describing the position of the genital 

 pores as follows : " Porus genitalis masculus inter annulos 25-26, vulva inter 

 annulos 27-28 hians." 



Testes (te., Figure 19), six pairs placed intersegnientally in somites 



xjn. _xvin. _ rpj^g terminal part of the vas deferens (ejaculatory part) is un- 

 XIV. xix. 



