THE LEECHES OF MINNESOTA 91 



body, were detected. Undoubtedly suitably preserved material 

 would exhibit the sensillas typically distributed and essentially as 

 they occur in related species. Eyes are very difficult to detect in 

 surface views of preserved adults but small pigment masses occur 

 at 1 1 I/I V in the same position as in P. parasitica and (distinct 

 eyes appear at III/IV in the young). However, it has not been 

 determined whether the eyes are simple or aggregated. 



In spite of the obscurity due to great and often unequal con- 

 traction of the annuli a careful analysis of the external mor- 

 phology shows that, except for the caudal peduncle and an appar- 

 ently greater simplicity of corresponding anterior segments of P. 

 pediculata, the structure is essentially as in P. parasitica. In respect 

 to the annulation the condition existing in young leeches must be 

 accepted with some caution as the somites become increasingly 

 complex with growth and age. The annulation of somites I to 

 IV of adults is unknown but in the young (I and II contain each 

 but a single annulus and III and IV are biannulate). Somite V 

 is biannulate dorsally but ventrally the furrow fades away to- 

 ward the median line ; VI is triannulate at the margins but the fur- 

 row al/a2 is incomplete above and even more so below. Somites 

 All to XXIII (or XXIV) are triannulate but the furrow al/a2 is 

 incomplete medially on the venter of both VII and VIII and on 

 most of the succeeding somites is less marked than either a2/a3 or 

 the intersegmental furrows. On anterior somites, and, to a less de- 

 gree on the posterior a3 is slightly longer than al or a2. 



The annulation of the post-anal somites, constituting the 

 caudal peduncle, is irregular and somewhat puzzling on the adult 

 specimens, but here also most of the somites, while very short, 

 appear each to be made up of three small annuli of varying size 

 and incompletely defined limits. Figure 16 represents accurate- 

 ly the exact arrangement of the furrows. On young specimens 

 (somite XXIV is triannulate, XXV, XXVI and XXVII are all 

 biannulate but al of somite XXV is partially divided and al of 

 both XXVI and XXVII is larger than a2). Somite XXIV, which 

 immediately succeeds the anus, is the last segment of the body 

 proper and on the contracted specimens its posterior border forms 

 a fold which envelopes the contiguous portions of the narrow ped- 

 uncle. The latter continues to taper to the sucker, to the middle 

 portion of which it is strongly attached for rather more than the 

 posterior half. The posterior sucker is large, circular and directed 



