264 SOME MINUTE ANIMAL PARASITES 



cases its presence has not been proved, ^but it is 

 always minute, and during some phases of the life- 

 history of the organism is not easily detected. 



A conspicuous feature of some members of the 

 group is the contractile vacuole. While some 

 Ciliates have but one vacuole, usually two or more 

 are present. They contract rhythmically, and col- 

 lapse instantly when they reach the surface. The 

 vacuoles may occur at any position in the organism, 

 but the vacuole of Blepharocorys (Fig. 49, c) discharges 

 direct into the anus. 



Defensive structures are not common among the 

 Protozoa as a group, but they are present in the 

 free-living Ciliates and to some extent in the para- 

 sitic ones. The outer layer of the body has a series 

 of small pits in it, and within these pits are threads 

 which can be discharged when necessary. They 

 seem to be both defensive and of use in food capture. 

 Blepharocorys has a long filament (Fig. 49, h) used 

 generally for attachment, but possibly defensive. 



Multiplication among Ciliates is abundant. Many 

 of them simply divide by binary fission over and over 

 again, until an enormous number of generations have 

 been accomplished. This asexual reproduction is 

 accompanied by decrease in size, and after a time 

 the daughter organisms become enfeebled and may 

 lose entirely the power to continue division. Gener- 

 ally, before this occurs two individuals form a tem- 

 porary association for the purpose of exchanging 

 micronuclear material, the result of which is to 

 revitalize both members, enable them to attain full 

 size again, and once more propagate by division. It 



