366 GENERAL PRINCIPLES 



structures (haustoria) into the stem of the host and from its 

 tissues absorbs nourishment. Then it continues to grow, 

 cHmbing upon the host and winding about from stem to branch, 

 and from one plant to another, until a veritable tangle of golden 

 threads is spun about the hosts. The leaves of the dodder are 

 minute scales. It has no chlorophyll and its mode of nutrition 

 is wholly parasitic. Small white flowers are finally produced and 

 seeds as in normal holophytes. Our American ''mistletoe" 

 is only partially parasitic. 



752. Among animals we likewise find parasitism more com- 

 mon among lower forms. Among Vertebrates only the round- 

 mouth eel, the lowest of fish-like forms, deserves the name 

 parasite. The sponges and Echinodermes contain no parasites; 

 Ccelenterates and Molluscs very few. The unsegmented worms 

 are the largest contributors to the list, and Insects follow closely. 

 The Cestodes and Trematodes are the most common internal 

 parasites, and to the order Hemiptera belong most of the 

 external parasites. 



753. The Trematodes are in some respects comparable with 

 the rusts, especially with regard to the complicated life history, 

 multiplicity of methods of reproduction and tendency to alter- 

 nate hosts. As an example often described we may take the 

 hver fluke. 



754. The adult fluke lives in the liver of the sheep and matures 

 many thousands of eggs, which pass down the bile ducts into the 

 intestine, and thus to the exterior. From these eggs hatches 

 a free-swimming larva, provided the eggs, washed by the 

 rain, or by some other means, reach a pond or stream. The 

 larva has a pair of eye spots, but is otherwise very simply 

 organized. Its further development depends upon its coming 

 in contact with a certain species of fresh-water snail. This pro- 

 vided, it attaches itself and bores its way into the interior 

 of the snail, where it continues to develop as a parasite. It 

 loses eyes and cilia (sense organs and locomotor organs) and is 



