PARASITES AND THEIR DEVELOPMENT. 191 



veto the belief. The lowest animals appeared first, and 

 were succeeded by forms of gradually increasing complexity. 

 Hence the parasites must have been developed before their 

 hosts. Man appeared long after the tapeworms or their 

 ancestors were produced ; and the intricate relationship be- 

 tween man and his neighbour-animals and the parasites must 

 have been acquired in a gradual fashion. Best of all, this 

 opinion is supported by the information to be gained from a 

 survey of parasitic life at large. We may begin such a sur- 

 vey by noting animals which attach themselves to other 

 animals as mere "lodgers." Such are external parasites. 

 Next may be traced parasites which depend for house-room 

 upon other animals, but which do not require board and 

 sustenance from their hosts. Such " messmates " are pre- 

 sented by the little fishes which live within the bodies of 

 large sea-anemones and of other organisms, and which swim 

 in and out at will, obtaining their food for the most part 

 from the external world. A simple modification of habit in 

 such animals would convert them into true parasites. Sup- 

 pose that the guest found that it might readily obtain food 



FIG. 20. Sacculina purpurea, a crab-parasite, showing its "roots." 



by living on the matters its host elaborated for its own use, 

 and suppose, further, that the animal-guest gradually accom- 

 modated itself by successive modifications to its new mode 

 of life, we have thus the influence of habit brought into 

 play and exercised upon the descendants of the first parasite 

 in producing a literal race of such beings. Such a belief or 



