4-22 
ENDO-PARASITES. 
'L'rue parasites are entirely (lependeiit upon, and unable to exist 
apart from, their hosts, as their alimentary canal has become degener- 
ate and their nourishment is derived solely from the fluids of their 
host, which are constantly being absorbed through the general inte- 
gument and assimilated by the animal independent of its will. 
Mutualists, commensals, or messmates, however, only avail themselves 
of the shelter and protection that close proximity affords, feeding 
independently of or in company with their entertainer. 
Parasites, according to their habits and modes of life, may, for our 
purpose, be divided into three sections, Ecto-parasites, Endo-parasites, 
and Commensals. 
The Enuo-Parasites of the mollusca are chiefly drawn from the 
Trematodes, a large number of whose species pass through one or more 
of the intermediate stages of their life’s cycle within the bodies of 
various mollu.sks, many attaining their perfect or sexual development 
within the body of some vertebrate ; each species is, however, more 
Fig. 7:i3. 
Fig. 73i. 
Fig. 73.5. 
Fig. 736. 
Fig. 737. 
Fasciola hc/>atica^ an endodermic parasite, showing the successive stages through which this 
species passes before attaining sexual maturity (highly magnified, after Marshall and Van Beneden). 
Fig. 733. — Free ciliated embryo. 
Fig. 734.— Sporocyst, as living usually in lung cavity of Livinoca truncatula. 
Fig. 735.— Redia, as living amongst the viscera and especially within the digestive gland of the 
mollusk into which it has migrated from the lung cavity. 
Fig. 736. — Cercarian stage which has escaped from the mollusk and again become free, eventually 
encysting upon the grass and being eaten by sheep. 
Fig. 737. — The mature sexual Distonia^ as developed from the cercarian stage within the bile 
ducts of sheep. 
or less restricted not only to particular species, but to special or 
determinate parts or organs thereof. 
These flat or soft worms usually begin existence by following a free 
aquatic life, travelling by the aid of a ciliated investment and eventu- 
ally effecting a lodgment within the body of the first suitable animal 
