94 THE LEECHES OF MINNESOTA 



Placobdella hollensis (Whitman) 

 (Plate II, fig. 11) 



Clepsine hollensis Whitman (1892) 



Description. This very distinct species will retain a perma- 

 nent interest for zoologists because of its having furnished the 

 material for Whitman's classical analysis of the nervous system 

 of the leech. Its place in the fauna of Minnesota is established by 

 several examples taken in Poplar Lake near St. Paul and sent to 

 me with other leeches by Prof. Henry L. Osborn. 



The form is very similar to P. parasitica but the present 

 species is a very much smaller leech, a length of from one to one 

 and one-half inches being about the usual size, though individuals 

 reaching two inches in extension have been observed. The most 

 obvious external characteristic is found in the eyes. As in other 

 species of Placobdella a pair of large contiguous compound eyes 

 exists in somite III with their bases resting in a conspicuous pig- 

 ment mass' and their principal visual component directed forward. 

 But unlike the other species described this pair is succeeded by an 

 indefinite number of pairs of much smaller eye-like organs which 

 Whitman has shown to be the modified dorso-median sensillae, 

 which possess a diminishing number of visual cells in each succes- 

 sive pair toward the caudal end, and gradually pass into the ordi- 

 nary sensillae. Superficially each appears as a small clear or whitish 

 area anterior to which more or less black pigment is accumulated 

 in the form of an irregular cup. The first pair (on IV) is decidedly 

 prominent and those on V and VI are also quite conspicuous and 

 eye-like. At first sight, therefore, this might be described as an 

 eight-eyed leech, with the first pair of eyes directed forward, the 

 remaining three, which are smaller and simple, backward. More 

 careful examination shows that the same features exist in a lessen- 

 ing degree in several additional pairs of the dorso-median and some 

 of the dorso-lateral sensillae as well, making it quite impossible to de- 

 termine just where the visual possibilities of the sensillae cease. All 

 of the sensillae are very distinct, rendering this a very favorable object 

 for study on this subject. 



The back is more or less roughened with small sense organs 

 and a few larger round smooth papillae. The latter correspond to 

 the largest papillae of P. rugosa and are most prominent posteriorly. 

 In the Minnesota specimens they begin on the neural annulus of 



