398 



GENERAL BIOLOGY 



The two primal functions of feeding 

 and reproduction not even the parasite 

 may lose; on the contrary it often 

 develops improved feeding apparatus 

 and increased reproductive capacity; 

 sacculina has done so; and the liver 

 fluke, which is parasitic on two hosts, 

 snail and sheep, at different stages of its 

 existence has developed an extraordinary 

 reproductive capacity to meet the exi- 

 gencies of shifting from one host to the 

 other. 



Parasitism may be either external or 

 internal, temporary or permanent, at one 

 stage, or during the whole life of either 

 host or parasite, on the part of the female 

 only (in its incipiency, the female seek- 

 ing shelter for her brood) or on the part 

 of both sexes. 



Parasitism is one of many possible 

 shifts for a living. The opportunities 

 for it have lain in the accumulation of 

 stores of rich organic products on the 

 part of the larger organisms. These are 

 available only to smaller species. Hence 

 parasitism is a prevalent habit mainly 

 among the smaller organisms. The 

 larger parasites offer like opportunities 

 for smaller ones, and are themselves 

 parasitized. The common bittern has 

 FIG. 233. Indian as an external parasite the fly shown in 

 Stic 'ffow e ering S P P iant" fig. 2 3 4 , living among its feathers. The fly 

 has its own external parasites the mites 

 show in the. figure, clustered at the joints of the legs, where 



