4G ZOOLOGY. 
into rings, and bearing some resemblance to a leech, but sometimes so short 
as to be circular. i 
‘1. The Zrematoda are internal monoicous parasites, having an anterior, 
suctorial opening, and one or more suctorial disks of attachment, which 
afford generic characters. Distoma (or Fasciola) hepaticum (pl. T, fig. 36), 
which is a good example, is about an inch long, infests the gall-bladder, 
liver, and rarely the neighboring veins in man, sheep, oxen, deer, gazelles, 
camels, goats, horses, and hares. In sheep it is the cause of the fatal 
disease named rot. The severe winter of 1841-2 in Germany, was fol- 
lowed by the death of many deer, which were found to be much infested 
with Distoma. 
Fresh-water snails of the genera Planorbis and Limnea are infested by a 
minute animal, with a globular body and slender tail, resembling a tadpole, 
and forming the supposed genus Cercaria, of which two American species 
were published in 1840, the motions of which are similar to those of their 
European analogues, Bic tail being rapidly thrown into the shape of an §, 
and easily detached.* Steenstrup has in some measure cleared up the his- 
tory of the Cercarize, which are the larvee of Trematoda. After swimming 
about freely for some time, they attach themselves to the outside of the 
snail, and settle in the mucus of the exterior, maintaining themselves by an 
abdominal sucker, and in the course of their movements losing the tail, a 
loss which gives them somewhat the appearance of a Distoma. They now 
enter the pupa state, in which they remain for some months without appar- 
ent change. They afterwards acquire spines anteriorly, and such individu- 
als were found within the snail. C. Th. Siebold thinks the Distoma is 
finally developed in the water-fowls which swallow the snails, an analogous 
tact having been observed by Creplin, who found a species in a stickleback 
fish, and also in water-fowls. 
2. The Planarvide contain a number of small leech-like animals, found 
both in fresh and salt water, which glide along like a snail over solid objects, 
or, passing up an object to the surface, they creep along this with the back 
downwards, and the belly attached to a thin film of water. The single 
opening to the ramifications of the stomach is usually about the centre of 
the inferior surface; and whatever is taken through this that is indigestible, 
is subsequently rejected by sucking in a quantity of water, and ejecting the 
whole together. 
Planaria (Planocera) cornuta (pl. TT, jig. 35) has two  horn-like 
extensions anteriorly. P. (Dendrocelum) gracilis, Hald. 1840, is three 
fourths of an inch long, and one tenth broad, fuliginous, veined with black ; 
* (©. hyalocauda, Hald. Body dark brown, or blackish, about as long as the tail; tail trans- 
parent, tapering, and suddenly diminished at its junction with the Peay Just patie to the 
naked eye. Parasitic upon Physa heterostropha. Susquehanna. 
C. bilineata.. Perfectly white. Microscopic characters : Head and tail translucent ; body with 
two dark longitudinal lines, which have a tendency to connect, so as to form a circle when the 
animal contracts; there is a light posterior circular spot occasionally visible, the tail is shorter 
than the extended body, and is not contracted at the base. Exceedingly numerous upon speci- 
mens of Limnea catascopium, collected at Camden on the Delaware. 
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