34 THE LEECHES OF MINNESOTA 



depression has a much larger diameter. A look at Plate C will 

 help toward a clear conception. 



PI. C, Fig. 5, shows the holes in the isthmus of a fish from 

 which the leeches have been removed, and Fig. 6 represents a 

 longitudinal section of one of these holes. The surrounding tis- 

 sue of the fish rolls up to form an inflamed collar about the attenuat- 

 ed peduncle of the leech. These leeches are capable of becoming 

 greatly contracted and when one is disturbed it draws back until it 

 appears as a mere brownish pyriform knob which entirely covers 

 the place of attachment. This 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 posi- 

 tion between somites XXIII and XXIV instead of between somites 

 XXVII and XXVIII as in other members of this genus. It is no- 

 ticeable 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 (Fig. 4), showing none of the pedicular 

 characteristics so pronounced in the older individuals. The pos- 

 terior sucker, however, is very strongly developed even in those not 

 more than a centimetre long. 



Practically nothing is known of this leech apart from its host, 

 but it seems possible that a part of its existence may be spent else- 

 where. During September 1903, I examined several thousand fish 

 of this species from Lake Pepin and found only three isolated 

 leeches, each about a centimetre 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 flesh. As full grown 

 specimens, deeply attached, 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. 



