DK. THEODOK NEUMANN. 215 



stances, local conditions, etc., make one species as hosts 

 more or less necessary ; thus it will happen that certain 

 animals become the principal hosts of others, the latter 

 being their regular parasites, though this still may not 

 yet be permanent, but only temporary, the one host be- 

 ing visited merely from time to time for the purpose of 

 getting food. Nevertheless, here we have the beginning 

 of a purely parasitic manner of life ; the continuation is 

 not hard and follows soon. It cannot be denied that 

 subsistence by means of nourishment which is prepared 

 by other animals, ready for use, offers many advantages 

 over that which is secured by the process of catching 

 prey and preparing it for digestion and assimilation. If, 

 therefore, an animal finds a possibility of continuing 

 such a cheap way of providing food, it will readily adopt 

 it, although the adoption mast be paid for often very 

 dearly in other ways. 



Hand in hand with this goes, of course, an adaptation 

 of the organs which serve the i)urpose of drawing food 

 from another animal. We shall discover a completion 

 and perfection of sucking organs, of clinging apparatus, 

 or, if there were none before, a new formation of such. 

 Then it may haj^jpen that the parasite loses completely 

 the ability to obtain food in the ordinary way and can 

 satisfy its needs only in parasitic fashion. It becomes 

 entirely dependent upon a host, often upon a specific 

 host, and is therefore forced to stay near it at all times. 

 When located on it, it will not like to leave it, at least 

 except for special, urgent reasons ; later not at all, the 

 parasite thus becoming stationary, permanent. 



Still it may have its seat only on the outside of the 

 host, but from there to the interior is only a short way. 

 The parasite will soon look for more protected parts of 

 the body in order to be less molested by outside infiu- 

 ences, and the mouth or the gill-openings, the outer ear 

 of animals, will be welcome hiding places. From 



153 



