40 BULLETIX: MUSEUM OF COMPAEATIVE ZOOLOGY. 



Length, fully extended, 25 mm. ; partially contracted (as in Figure 21), 

 about 10 mm. 



JJ'idth, fully extended, less than 1 mm. ; partially contracted (as in Figure 

 27), about 1.5 mm. 



Color. — The anterior and marginal parts of the body are very clear and 

 transparent. The rest of the body is usually of a pale yellowish-white color 

 when the animals are first collected, but changes to a rusty yellow or pale 

 orange color if they are kept in well-lighted aquaria for a few days. The color 

 is due to the presence in the deeper parts of the body of rounded reserve-food 

 cells, similar to those described as occurring in G. stagnalis. Apparently the 

 nature of the granules in the reserve-food cells changes under the influence of 

 daylight, so that by reflected light they appear pale orange instead of yellowish- 

 white, the color which they have when first collected. 



Superficial pigment cells of the branched type, described as occurring in G. 

 stagnalis and other species, appear to be entirely wanting in G. elongata. 



Fat cells occur in abundance in the deeper parts of the body, the contained 

 oil drops being perfectly clear and transparent, as in G. stagnalis and G. fusca. 



b. Rings, Somites, Eyes, Suckers. 



The skin is very smooth and entirely free from papillae. 



External rings, broad and smooth, usually indistinct in the head region 

 (somites l.-iv.. Figure 23). Number of rings, 62 between oral sucker and anus 

 (somites v.-xxvii.). 



Notwithstanding the indistinctness of the rings in the head region, favorable 

 preparations, like that represented in Figure B, show that the composition of 

 somites i.-iv. is practically the same in this species as in G. heteroclita (Plate 5) 

 and G. fusca (Plate 4). Somites I. and ii. are uniannulate ; somites ill. and 

 IV. biannulate, the anterior rings being broader and corresponding to rings 1 and 

 2 of a typical somite taken together. 



Somite v. is likewise biannulate in this species, just as in G. stagnalis 

 (Figure B ; compare Plate 1, Figure 3) ; in all the other species with which 

 this paper deals, somite v. is triannulate. 



Somites vi.-xxiv. (Figure 27) are triannulate, as in all other known species 

 of this genus. Somites xxv.-xxvii. are reduced each to a single ring, a con- 

 dition found in the other species described only in the case of somite xxvii., 

 somite xxv. being always biannulate, and somite xxvi. usually so. 



Eyes, two, situated about as in G. stagnalis, just posterior to the mouth, 

 between somites in. and iv. (Figure 23). The eyes are separated from each 

 other by a considerable space, as in G. stagnalis (Plate 2, Figure 4) and G. fusca 

 (Plate 4, Figure 16). The pigment associated with them is usually small in 

 amount ; often it is wanting altogether. 



The oral sucker, as in the other .species described, lies within tlie limits of 

 somites i.-iv. The mouth Vies about in its centre (Figure 23, Plate 6 ; Figure B). 

 The posterior sucker (act.. Figures 24, 27) is extremely small and weak. In 



