INTRODUCTION 



In the summer of 1899, while at Lake Pepin superintending- the 

 zoological work of the Geological and Natural History Survey of 

 Minnesota, Professor Nachtrieb found that some of the sheepshead 

 (Aplodinotus grunniens) which were being seined from the lake in 

 large numbers by the local fishermen, had a large parasitic leech 

 fastened to the isthmus or shoulder under the gill cover. Three of 

 these leeches were collected at that time, with portions of the fish, 

 showing the place and manner of attachment. One of these speci- 

 mens was later sent to Professor J. Percy Moore who found it to be 

 a new species of Placobdella and named it P. pcdiculata. All the 

 specimens originally collected were adults, gorged with blood, and 

 greatly modified in form from the usual Placobdella types by their 

 close parasitic habit ; so that, in some parts, annulation and many 

 other external features had been entirely obliterated. It was seen 

 at once that to determine these features, younger and better pre- 

 served material must be obtained, so during the first part of Septem- 

 ber 1903, I spent several days with the fishermen around the head 

 of Lake Pepin examining fish for these leeches. During this time 

 I examined many hundreds of fish and succeeded in obtaining three 

 small specimens, none of which were over a centimetre in length. 



Methods. 



The leeches were removed from their hosts as soon as found 

 and placed in carbonated water (I used the ordinary bottled "pop" 

 for this purpose) in which they soon became fully extended and 

 stupefied. After they had become perfectly quiet (in five to ten 

 minutes) they were transferred from the carbonated water to Gil- 

 son's mercuro-nitric mixture, in which they were left for an hour, 

 and then put into 80% alcohol and treated with iodine in the usual 

 way. This method left all well extended and in excellent condition 

 for study. One was stained in bulk in Mayer's Paracarmine, im- 

 bedded in paraffin, cut in transverse series 20/1000 mm. thick, and 

 mounted without further staining. The second was stained in bulk 

 in Mayer's Paracarmine, imbedded in paraffin, cut in sagittal series 

 5/1000 mm. thick, and counterstained with Lyons blue before 



