52 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



XXVI. are commonly divided at the margin of the body into a broad anterior 

 and a narrow posterior portion. Somites ill. and iv. are biannulate, the broad 

 anterior ring in each case bearing the sensillse and representing both the an- 

 terior and the middle ring of a triannulate somite (Figure 2, iii.-vi.). The 

 remaining preanal somites (v.-xxiv., Figm-e 6) are triannulate, but the pos- 

 terior annulus of xxiv. is narrower than the adjacent anuali (Figure 6), and 

 the anterior and middle annuli of somite v. are united ventrally while sepa- 

 rated by only a very shallow furrow dorsally (7, 8, Figures 2, 3 b, Plate 1. 

 These two cases illustrate the centripetal progress of abbreviation (or arrested 

 development), that part of each terminal triannulate somite being affected 

 which is adjacent to an abbreviated somite. 



In Figure 32, Plate 8, is shown a rather unusual condition, the apparent 

 disappearance of the furrow separating somites ii. and ill.^ 



The total number of preanal rings is sixty-nine, counting somites I., ll., 

 and xxv.-xxvii. as uniannulate, iii. and iv. as biannulate, and v.-xxiv. as 

 triannulate (Figure 6). 



c. Eyes, Mouth, Oral Sucker. 



The eyes appear in the living animal, or in whole preparations, as a single 

 pair closely united and situated in rings 3 and 4 (somite ill.). See Figure 6, 

 Plate 1 ; and Figures 32, 33, Plate 8. An examination of sections, however, 

 particularly of young individuals, shows that there are really three distinct 

 pairs of eyes present, there being a small rudimentary pair anterior, and an- 

 other still more rudimentary posterior to the principal pair of eyes, exactly as 

 shown for " C. hollensis " by Whitman ('92, Figure 6). 



All three pairs of eyes^ are partially imbedded in a common pigment mass, 

 the anterior and middle pairs being directed forward, the posterior pair back- 

 ward, just as in G. elegans and G. heteroclita (Figures 20, 29). The largest 



^ A similar condition is figured by Whitman ('91») in his Plate 15, Figure 1. In 

 his text, however, Wiiitman says (p. 412) : " In front of the eyes I was unable to 

 discover any distinct rings. In another species C. chelydnv, from Wisconsin, tliere 

 are three narrow rings in front of the eyes ; and the first is marked by the usual 

 nietameric sense-organs. Although no metameric sense-organs were recognized 

 in front of the eyes in C. plana, the correspondence of other metameric characters 

 in the two species is suflBcicntly close to enable rae to identify the ocular rings as 

 equivalents. The preocular part of the head is, therefore, probably equivalent to 

 the first somite of C. chelydra;, and is so numbered in Figure 1." 



In view of Whitman's subsequently published studies on " The metamerism of 

 Clepsine " ('92), I think he unquestionably would now recognize two preocular 

 somites both in " 0. plana" and in " C. chelydra; "; at any rate, that is the numlier 

 found in the species which I am describing (Figure 2, I'late 1). Since Whitnuui 

 has pointed out no other difference between his "plana "and " clielydrae " than 

 the uncertain one of preocular rings, I consider that their specific distinctness 

 remains to be estabiiBhed. 



2 Only the largest (middle) pair of eyes appear in the section shown in Figure 2. 



