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 



