334 FOUNDATIONS OF BIOLOGY 



find their normal habitat in the digestive tract, where, inci- 

 dental to getting their own living, they bring about chemical 

 changes in the food of their host which is an important factor 

 in the digestive processes of the latter. 



Still another type of association in which both partners 

 profit is represented by the relation that occurs between Ants 

 and Aphids. The defenseless Aphids are protected, herded 

 and 'milked' by the Ants to supply their demand for honey- 

 dew, a secretion of the Aphids which the Ants greedily de- 

 vour. (Fig. 180.) 



3. Parasitism 



But associations in which one organism, the PARASITE, 

 secures the sole advantage, and in most cases at the expense 

 of the helpless second party, the HOST, are far more numerous 

 it has been estimated that nearly half the animal kingdom 

 are parasites. And these are particularly forced upon our 

 attention because many human diseases are the result of 

 Man's unwilling partnership in such associations. Indeed, 

 PARASITOLOGY has become an important subdivision of bi- 

 ology, both practical and theoretical. Practical, as a corner 

 stone of public health; and theoretical, because many of the 

 most remarkable functional and structural adaptations are 



FIG. 181. Diagram illustrating the life history of a Malarial Parasite. The stages 

 above the line of dashes occur in human blood; those below, in the body of a Mosquito. 

 I-V and 6-10 show asexual multiplication (schizogony) in human red blood corpuscle 

 following introduction of a parasite (XIX) by a Mosquito. This may continue by the 

 parasites (10) entering other corpuscles until a large number of the latter are destroyed. 

 Sooner or later sexual forms arise. VI-XIII, the sexual generation involving the 

 differentiation of male ( $ ) and female ( 9 ) gametes which unite (XI) to form a zygote 

 (XII). The zygote becomes motile (XIII), works its way into the wall of the stomach 

 of the Mosquito, and encysts (XIV). Within the cyst a number of small cells (XVI, 

 sp.bl.) arise by division, and these, in turn, give rise to a multitude of motile cells 

 (XVIII) termed sporozoites. The sporozoites are liberated (XIX) from the cyst, 

 make their way to the salivary glands of the Mosquito where they are ready to be 

 inoculated into the human body, and so gain entrance to a red blood corpuscle (I). 

 The production of the sporozoites from the zygote is known as sporogony. n, nucleus 

 of the oarasite: p, pigment and waste products of the parasites; fl, long slender male 

 gametes. (From Minchin, in Lankester's Treatise.) 



