No. 500] PL A C OBDELLA PEDICULATA 



529 



The burying of the posterior segments in the tissues of 

 the host has brought about an interesting structural 

 change so that we find the anal opening shifted forward 

 to a position between somites XXIII and XXIV instead 

 of between somites XXVII and XXVIII, as in the other 

 members of the genus. It is noticeable that, while the 

 young leeches whose posterior portions are not yet 

 deeply imbedded have the characteristic position of the 

 anus (XXIII-XXIV), the outline of the posterior part 

 of the body is still a regular curve showing none of the 

 pedicular characteristics so pronounced in the older in- 

 dividuals. The posterior sucker, however, is very 

 strongly developed even in those not more than a centi- 

 meter long. 



Practically nothing is known of this leech separate 

 from its host, but it seems possible that a part of its 

 existence may be spent elsewhere. During September, 

 1903, I examined several thousand fish of this species 

 from Lake Pepin and found only three isolated leeches, 

 each about a centimeter in length. The posterior sucker, 

 while imbedded in the tissue, was not sunk in deeply and 

 so had not produced the characteristic peduncle. They 

 were evidently young ones which had recently attached 

 themselves to their hosts and were gradually sinking the 

 posterior sucker into the host's flesh. As full grown 

 specimens, deeply imbedded, were found in the same 

 locality during August of 1899, at least some of the 

 adults must remain with their hosts during the summer 

 and probably throughout the year. 



Description 2 



Like Placobdella parasitica and P. rugosa, this is a 

 species of large size, though not quite equaling the largest 



2 This description is based upon both young and large mature speci- 

 mens gorged with blood. In view of unavoidable 'delay in the publication 

 of Professor Nachtrieb's projected report on the leeches of Minnesota, 

 Professor Moore kindly consented to the free use of his description pre- 

 pared for the systematic portion of the report here alluded to. I have 



