246 ON THE BLACK PULP LEECH. 



that accuracy which should mark the progress of science, their super- 

 ficial observations concerning it have led to those numerous errors which 

 darken the history of the whole genus. 



Sometimes it is found in slow-running waters, and there of the largest 

 size ; but usually when they are almost stagnant, and, in particular, 

 where aquatic plants abound. On these, numbers crowd together as the 

 colder season advances, each contracting into an elliptical spot, and then 

 affording the most favourable opportunity for many being taken with 

 facility at a time ; or they fix on the under surfaces of pieces of slate 

 and smooth stones, and somewhat elevated above the mud. They live 

 in numerous pacific societies, associating with every other species, 

 and are so generally dispersed, that none has occurred to me of such 

 frequency in Scotland. 



An animal so well known as the Pulp Leech, (Planaria,} scarcely 

 requires description ; nor considering its external uniformity of appear- 

 ance, does there seem much room for commentary. It is found of all 

 different sizes, from extreme minuteness to above half an inch in length, 

 and about an eighth of an inch in breadth. When very large, it has a 

 dull and sluggish aspect, and is chiefly roused to action by light and 

 heat. On the under part; or belly, are what naturalists have designated 

 two ventral pores, one of which, perhaps both, is more conspicuous at 

 certain seasons. The first is lower down than the middle of the body, 

 and the second still nearer the extremity of the tail. The opacity of 

 the animal precludes us from accurately distinguishing its internal orga- 

 nisation ; nevertheless, enough may be discovered to infer that it is far 

 from simple. Among Pulp Leeches, (Planaria,) some occasionally 

 occur of the ordinary size, figure, and proportions ; but, instead of the 

 jetty colour proper to the species, they are of a dark grey. By inter- 

 posing them between the eye and concentrated rays of light, the internal 

 structure may be obscurely recognised in many vessels branching from 

 about the middle of the body, and terminating in obtuse extremities 

 near the margin. These are certainly analogous to the pinnate ramifi- 

 cations already described ; and they bear much resemblance to the viscera 

 of leeches. Facts do not warrant our concluding, that the Grey Pulp 

 Leeches, (Planarice,) are indubitably a different species, or that they 

 are only varieties ; their contour seems more ovate, indeed, and they are 

 seldom, if ever, seen of the largest size. One taken in October was of 

 light grey, and mottled or streaked with deeper transverse bands ; it 

 was very broad in proportion to its length. Yet it must be admitted 

 that repletion always affects the colour of the leading species, which 



