50 ANIMAL FORMS 
For this reason an almost incredible number of eggs is laid, 
and some extraordinary measures are employed in effecting 
the desired result. Probably the best-known example is that 
of the liver fluke inhabiting the bile-ducts in the sheep. 
Each worm lays several hundred thousand eggs, which make 
their way from the host, and if they chance to fall in pools 
of water or damp situations may proceed to develop, other- 
wise not. If the surroundings be favorable, the young, like 
little ciliated Infusoria, escape from their shells and rest- 
lessly swim or-move about for a short time, and if during 
this time they come in contact with certain species of 
snails living in these situations they at once bore into their 
bodies. Here they produce other young somewhat resem- 
bling a tadpole, that now make their escape from the snail. 
In a short time each one crawls upon a blade of grass, and 
surrounds itself with a tough shell, where it may remain for 
several weeks. If the grass on which they rest be eaten by 
a sheep, they finally make their way to the bile-ducts and 
there become adult. The life cycle is now complete; the 
young form has found a new host; and the process shows 
how wonderfully animals are adapted to the conditions which 
surround them, and how closely they must conform to these 
conditions in order to exist. 
49. The tapeworms (cestodes),—The cestodes, or tape- 
worms, are also parasitic flatworms in which the effects of 
such a mode of life are strongly marked. They occur 
almost exclusively in the bodies of vertebrate hosts and 
exhibit a great variety of bodily forms, in some cases resem- 
bling rather closely the trematodes, but in others strikingly 
different. In the latter type the body is usually of great 
length (from a few centimeters to upwards of sixteen meters 
(50 feet) ), and terminates in a “head” (Fig. 31) provided, 
in the different species, with a great variety of hooks and 
spines and numbers of suckers for its attachment to the 
body of the host. From the head the body extends back- 
ward in the gradually enlarging ribbon-like body, slender at 
