308 MANUAL OF ELEMENTARY ZOOLOGY 



gut, and Trypanosoma and Filaria in the blood vessels and 

 lymph spaces, (ii) in the tissues, as Dracunculus and young 

 Trichinella, or (iii) in protoplasm, as the young trophozoite 

 of Monocystis, or that of the malaria parasite. Its life is 

 passed in the following conditions : (i) Food is plentiful, 

 needs no hunting, and is usually easy of digestion, (ii) there 

 are no enemies to be escaped, (iii) means of fixation {e.g. to 

 the gut- wall) are sometimes needed, (iv) provision must be 

 made for distribution from one host to the next, (v) the 

 surroundings are often devoid of free oxygen. We find 

 accordingly in internal parasites (i) an absence or poor 

 development of organs of locomotion, of defence and 

 offence, and of sense, (ii) simplicity in the organs of nutrition, 

 (iii) often the presence of organs of fixation (suckers, hooks, 

 etc.), (iv) many adaptations of the life-history to distribution. 

 Such are : (a) the vast numbers in which the young are 

 usually produced, so that some survive the "passage 

 perilous," (b) the occurrence of special distributive indi- 

 viduals, either active, as the miracidium of the liver fluke 

 and the larval stages of Tylenchus, or adapted to passive trans- 

 mission, as the gamonts of the malaria parasite, various 

 encysted Protozoa, and hard-shelled eggs of worms — thus the 

 parasite has a multiplicative phase in which it takes advan- 

 tage of the rich supply of food in its host to produce many 

 offspring, and a distributive phase in which temporarily 

 non-reproductive individuals find new hosts — (c) often the 

 passage through an intermediate host. When there are 

 two different hosts, the principal host is that in which the 

 parasite passes its sexual phase (not necessarily the larger 

 host, as is seen in the case of the malaria parasite). This host 

 usually preys on the intermediate host. 1 The intermediate 

 host of the malaria parasite is man, that of the pork tape- 

 worm is the pig, that of Filaria is the mosquito, that of the 

 guinea worm is the freshwater crustacean Cyclops. The 

 advantages of this arrangement are twofold : (ij the parasite 

 finds a means of re-entering its principal host, (2) very often 

 the intermediate host provides a richer supply of nourish- 

 ment than the principal host, and the parasite there under- 

 goes most of its growth and reproduces asexually. (v) In- 

 ternal parasites are often able to obtain the energy for their 

 1 Filaria bancrofti is an exception lo this. 



