44 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Somite 111., within the anterior part of which lies the mouth (or., Figure 20), 
is ordinarily biannulate, as are also somites Iv. and xxv. (Figures 19, 35, 36). 
But in 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 first 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 
IlI., 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 sensille of somites 11-1v. 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 sucker, formed by somites 1.-Iv. (Figure 20). 
Mouth (or., Figure 20), in the anterior part of somite 111., usually a little an- 
terior to the first pair of eyes. 
Posterior sucker, as in other species, slightly longer than broad (Figure 19). 
c. REPRODUCTIVE ORGANS. 
Male and female 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 intersegmentally in somites 
XII. | XVII. 
teers The terminal part of the vas deferens (ejaculatory part) is un- 
Vey eee 
