castle: north American RiiYNCHOBDELLiDiE. 28 



Somites i. and ii. are together represented by a single broad ring (Figures 3, 

 4), wLicli, however, is sometimes subdivided >)y a shallow furrow (Figure 7, 

 Plate 3). 



Somites iii. and iv. consist each of a single ring, the latter forming the pos- 

 terior boundary of the oral sucker (Figure 3, Plate 1 ; Figure 7, Plate 3). 



Somites xxv, and xxvi. consist each of two rings, a broad followed by 

 a narrow one (63 and 64, 65 and 66, Figure 4, Plate 2 ; Figure 34, Plate 8). 

 The narrow ring of somite xxvi., however, is often so completely fused with 

 the broader ring which precedes it as to be scarcely distinguishable. 



Somite xxvii. consists of a single broad ring, crowded back to a position 

 lateral and posterior to the anus (67, Figures 4, 34, and A). 



Somites xxviii.-xxxiv. are not represented by external rings ; in the 

 central nervous system, however, we shall find clear evidence of their separate 

 existence. A further discussion of the metamerism will be deferred until the 

 nervous system has been described. 



Eyes, two, large and distinct, lying in the anterior part of ring 3 and ex- 

 tending forward into the posterior part of ring 2 (Figures 4, 7).^ 



c. Dorsal Gland, Suckers. 



Dorsal Gland. — Between the twelfth and thirteenth rings (that is, between 

 the anterior and middle rings of somite vili.) on the mid-dorsal surface of the 

 avimal, is a structure {gl. d., Figures 4, 7) peculiar to this species, though accord- 

 ing to Apathy ('88^) its homologue is found in some other species, either as a 

 functional structure in the embryo, or as an inconspicuous rudiment in the 

 adult. It consists of a rounded, wart-like, yellowish-brown, cuticular plate, 

 often surrounded by a ring of substance similar but lighter in color, probably 

 because less well hardened. These structures are secreted by a patch of 

 high columnar epidermal cells, which in the embryo, according to Apdthy, 

 form a sort of byssus gland serving to attach the young to the under side of 

 the mother before the suckers at the ends of the body become functional. In 

 the adult the organ has no known function, though it forms a fiivorite place of 

 attachment for a certain colonial protozoon of the genus EpistyUs. 



do 80, however, without feeHng at all certain that the terms are strictly appli- 

 cable in all cases or even in a majority of cases. I have elsewhere (Castle, 1900) 

 expressed the opinion that the leech somite consisted primitively of a single ring. 

 If this is so, it may well be that the somites commonly spoken of as abbreviated 

 have really never attained the triannulate condition. (Moore, 1900, has expressed 

 a similar view since this paragraph was written.) Nevertheless the term is a con- 

 venient one to express deviation from the typical condition of the somite in the 

 direction of a shortening of it. In this sense the term will be employed in this 

 paper. 



1 Budge ('49) likewise represents the eyes in the anterior part of ring 3. 

 Apathy ('88"), however, counts the ocular ring the fifth, emphasizing subdivisions 

 which can occasionally be seen in the most anterior rings. (Compare his Figures 4 

 and 10 with my Figures 3 and 7.) 



