232 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



heron harbors an adult fluke, Clinostomum marginatum, in its mouth 

 cavity. Eggs discharged by the parasite reach the water and soon 

 hatch, the miracidia penetrating snails. After several generations 

 in the snail, fork-tailed cercariae emerge to penetrate under the 

 scales into the flesh and sometimes on the fins of many species 

 of fresh-water fish. Here they grow into the typical yellow grubs 

 commonly found surrounded by a cyst formed by the connective 

 tissue of the host. 



As a result of the above discussion of parasitism it is hoped that 

 some concept of the elaborate food chains and interrelationships and 

 interdependence characteristic of the various groups of parasites and 

 their hosts may be gained. Because these relationships are so com- 

 plicated and form so intricately woven a pattern, it becomes prac- 

 tically impossible "to predict the precise effects of twitching one 

 thread in the fabric." 



SUGGESTED READINGS 



Cowdry, E. V., et al., Human Biology and Racial Welfare, P. B. Hoeber, 1930. 



Ch. XVII. 



Popular discussion, resistance, etc., from the bacteriological point of 



view. 

 Elton, C., Animal Ecology, The Macmillan Co., 1935. Chs. V, VI. 



Excellent readable discussion of parasitism from an ecological view- 

 point. 

 Massee, George, Diseases of Cultivated Plants a>}d Trees, The Macmillan Co., 



1910, pp. 1-23, 59-77. 



A good discussion of parasitic plants. 

 Needham, J. G., Frost, S. W., Tothill, B., Leaf-Mining Insects, The Williams 



& WUkins Co., 1928. Ch. I. 



Deals with natural history of group. 

 Smith, T., Parasitism and Disease, Princeton University Press, 1934. 



Excellent general, but somewhat technical, discussion of the parasitic 



habit. 



