906 THE AMERICAN NATURALIST. | [Vor. XXXV. 
pp. 631—646, 1870) speaks of finding the worms in winter within 
Limnzea, and gorged with the kidney cells, while he seems to 
have found no such evidence of parasitism at other seasons. 
I should add that in one instance, in which the worms had 
been left for some time in a watch glass with a torn Physa, 
the alimentary canal of almost every one was found stuffed 
with the blackish pigment characteristic of the snail, which 
was floating in the water. 
The worm is a small, transparent animal, about 2 mm. in 
length and 1-5 mm. in breadth at its widest point. Both these 
measurements are taken in extension, but as the creature repro- 
duces by fission, and as colonies of at least three persons are 
common, the first impression is of a much longer organism than 
I have described. On the ventral side, near the anterior end, 
are two clusters of cephalic setze ; each seta is sigmoid, about 
1-14 mm. long, and ends in a fork whose prongs are equal and 
are bent nearly at right angles to the shaft; each cluster ordi- 
narily contains seven setze; though there may be six or, very 
rarely, five. In one case, one side bore the full number of seven, 
while there were but two on the other. It would seem probable 
that the remaining ones had been torn away. At some little 
distance behind these clusters are the abdominal ones, which 
differ from the former only in that the setze are smaller (about 
1-23 mm. in length), more numerous (being, as a rule, eleven, 
rarely ten, in a cluster), and borne-on a slight projection of the 
body which apparently corresponds to a rudimentary parapo- 
dium, This last peculiarity, combined with a habit of holding 
the hind end fixed while extending and waving about the ante- 
rior one, gives the worm, as Lankester has already remarked, a 
curious likeness to a geometrid larva. There are sometimes as 
many as thirteen pairs of seta clusters behind the cephalic one ; 
it is difficult to determine how many of these should be reck- 
oned as belonging to a single individual, but for reasons which 
will be given in discussing the budding, I am inclined to con- 
sider with Lankester that the adult has four pairs of abdominal 
clusters. 
The large mouth lies at the anterior end, but the dorsal edge 
projects rather over the ventral one, so that the tip of the body 
