THE 
AMERICAN NATURALIST. 


Vor. XXXIX. September, 1905. No. 465. 

INTERRELATIONSHIPS OF THE SPOROZOA. 
/ 
HOWARD CRAWLEY. 
OTHER things being equal, it is perhaps an easier task to 
determine the interrelationships of a group of parasites than 
those of a series of free-living forms. Once the parasitic habit 
is acquired, development is confined within narrow limits. Two 
definite lines are followed. These are: morphological degrada- . 
tion, and complication of the life history combined with increased 
fecundity. It is perfectly evident why this should be. Individ- 
ually, the parasite is at once relieved of two of the great prob- 
lems of animal existence. These are: the obtaining of food and 
the avoidance of serving as food for some other animal. The 
parasite has no enemies and abundant food surrounds it on all 
sides. As a result, various parts necessary to a free-living 
animal are lost. These are: the organs concerned with the cap- 
ture and digestion of food, and those which serve as protection 
against the attacks of enemies. On the other hand, the new 
mode of life involves new needs, and the organism responds by 
the production of new organs. Such are: hooks, providing for 
the maintenance of a fixed position, and, in many cases, beaks 
or rostra which enable their possessors to bore through the tis- 
sues of the host. 
607 
