SCOLECIDA. T2t 
only liable to be infested with the tapeworms derived from the 
cystic worms of other animals, but may be attacked by the cystic 
or immature forms of the tapeworms of other animals. Thus 
the disease known as “‘ hydatids” in the human subject is caused 
by the presence in his tissues of the cystic worms which are 
ultimately developed into the tapeworm of the dog. 
ORDER II. TREMATODA.—The “suctorial” worms, or “flukes,” 
as the members of this order are commonly called, are all inter- 
nal parasites, inhabiting various situations in different animals, 
but especially affecting birds and fishes. They are all more or ; 
less flattened and rounded in shape, and are furnished with one 
or more suckers, by which they adhere. They are distinguished 
from the Zeniada by always possessing an alimentary canal, 
which is often much branched (fig. 51, 1), is simply hollowed 
Fig. 51.—Trematoda. 1. Distoma hepaticum, the ‘‘liver-fluke,” showing the 
branched alimentary canal: 2. Anterior extremity of Distoma lanceolatum, 
enlarged ; a Anterior sucker; 4 Posterior sucker ; ¢ Generative pore ; d Gullet ; 
e e Bifurcating alimentary canal. (After Owen.) 
out of the tissues of the body, and is never provided with a dis- 
tinct anus. The best known of the 7vematoda is the common 
liver-fluke (Distoma hepaticum, fig..51, 1), which inhabits the 
gall-bladder or ducts of the liver in sheep, and is the cause of 
the disease known as the rot. In form it is ovate, flattened on 
the two sides, and presenting two suckers, of which the anterior 
is perforated by the aperture of the mouth. A branched water- 
vascular system is present, and opens posteriorly by a small 
aperture. The alimentary canal bifurcates shortly behind the 
mouth, the two divisions thus produced being much branched, 
and terminating posteriorly in blind extremities: In Dzstoma 
