EFFECTS OF PARASITISM. 



65 



Firmly fixed on or in the animal host, and relatively incapable of movement, the 

 animal parasite requires only feebly developed feeding and motile organs, or even 

 none at all, and accordingly no organs of sense. In animal parasites also, with 

 increasing parasitism, the external segmentation and the anatomical structure proper 

 to the parasite as a member of a highly organised group disappear more and 

 more; and here in general, just as in plants, the reproductive organs succumb 

 less than the vegetative organs to the destructive action of laziness. It is, in fact, 

 in both cases inactivity — laziness, which distinguishes parasitism, and effects the 

 degeneration of organs. As an animal which clings by means of suckers during its 



Fig. do.—Hydnora Africana. tt a small piece of the subterranean vegetative body of the host- 

 plant ; out of this spring a mature flower bl and flower buds bi' bl" (| natural size). 



whole life on or in another animal can dispense with the various activities which 

 other animals require for the seizing of their food, and the corresponding movements 

 and use of the sense-organs; so a plant loses, with the loss of chlorophyll, the 

 necessity for raising itself above the substratum to the light and of developing all 

 those adaptations which subserve this purpose. 



If we now turn to the more simply organised plants, the ]\Iosses and Algae, 

 we find, the lower we descend, continually simpler forms of shoot : we have to regard 

 these, however, not as reduced, but rather as rudimentary — as not yet typically 

 developed. Numerous transitional forms, however, indicate the passage from the 



[3] 



