318 BIOLOGY: GENERAL AND MEDICAL 



another's cost. In general it is a miserable form of ex- 

 istence characterized by many vicissitudes and marked 

 decadence of the parasitic forms. The actual decadence 

 of the parasites, however, depends upon the nature of the 

 symbiotic relationship. When this is disjunctive the 

 parasites maintain a certain degree of independence, 

 but when it is conjunctive they must live and die with 

 the host. 



The parasitic organisms include representatives of 

 both cryptogams and phanerogams among plants, and 

 of nearly all of the phyla of animals from the protozoa 

 to the vertebrata. 



According to their habits, they may be described as 

 occasional or optional parasites and obligatory parasites. 

 With the adoption of the parasitic mode of life struc- 

 tural modifications usually make their appearance so as 

 to adapt the organism to its environment, after which its 

 new mode of life becomes obligatory. 



Thus, the mosquito may be cited as an example of the 

 occasional parasite. Under conditions prevailing in 

 many localities where mosquitoes abound, these insects 

 live by sucking the nectar of flowers and the juices of 

 plants. They have, however, a marked preference for 

 the blood of animals and a few cannot ovulate except 

 after a meal of warm blood. 



The next step in the direction of obligatory parasitism 

 is seen among organisms that visit the hosts occasionally 

 though absolutely dependent upon them for the means 

 of subsistence. Among such we find the biting flies, 

 the fleas, and the bed-bugs. The latter form excellent 

 examples, for they do not live upon the host, but inhabit 

 crevices in the bed or cracks in the walls and sally forth at 

 night to feed. Their mouth parts are so constructed that 

 it is impossible for them to feed in any other way, and if 

 the host should vacate his habitation the bugs must 

 inevitably die, unless in the absence of human tenants 

 they can make shift with some other warm-blooded 

 animal that may become available. Fleas leap upon 



