676 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



Length, 18 mm. 



Length to male pore, 5 mm. 



Diameter of anterior sucker, i mm. 



Diameter of its opening, .6 mm. 



Diameter of posterior sucker, 2.5 mm. 



Diameter of its opening, 1.5 mm. 



Width of body at male pore, 3.8 mm. 



Depth at male pore, 2.4 mm. 



Greatest width (XIX), 6 mm. 



Depth at XIX, 4 mm. 



Width at anus, 2.5 mm. 



Depth at anus, 1.2 mm. 



Annulation. Somites I and II are found in the preocular lobe, across 

 which a very faint furrow can usually be detected and is always quite 

 evident in the larger specimens. Ill is biannulate, the first annulus 

 larger. IV is also biannulate, the anterior annulus bearing the single 

 pair of eyes, which are generally lightly pigmented and consequently in- 

 conspicuous. This somite is usually a trifle longer than III, but in some 

 examples they are equal. The example figured is slightly abnormal, the 

 left half of the furrow IV/V being displaced on the dorsal side. V is 

 likewise biannulate, but is considerably longer dorsally, where the enlarged 

 anterior annulus is quite distinctly subdivided by a furrow, which separates 

 a smaller ai from a larger a2. This furrow ends at the margins of the 

 lips so that ventrally there is no trace of this subannulation (Plate L, 

 figs. 17, 18 and 22). 



Although VI must be reckoned as a triannulate somite both above and 

 below, it does not show the typical condition, inasmuch as ai and a2 are 

 rather closely united on the ventral side, and aj is the only annulus 

 which exhibits any trace of that development of secondary annuli which 

 is so characteristic of this species among its close allies, and even this 

 trace is usually wanting. 



Somite VII is the first typical one. The three annuli are of about 

 equal size and each is usually distinctly subdivided into two nearly equal 

 secondary rings by a cross-furrow, which is especially well-marked on ai 

 and aj, and much less distinct or even absent ventrally. These cross- 

 furrows correspond in position with superficial annular lacunae which 

 encircle each primary ring of the complete somites, and they have a con- 



