4G ENCYSTMEXT OF THE PROTOZOA 



sometimes fifty or a hundred times as long as the spore itself, can be 

 extruded. The spores of the Cnidosporida are developed in a complicated 

 manner from a group of cells, some of which form the polar capsules. 

 These possess a tough outer covering within which the coiled-up filament 





Fig. 30. — Dark Field View of Spore of Nosema apis with Extruded Polar 

 Filament ( x 1,200). (After Kudo, 1921.) 



can be seen. It is supposed that the filament is inverted in the capsule, 

 and that it is discharged by pressure from within, just as an inverted 

 finger can be everted by blowing into a glove (Fig. 312). 



ENCYSTMENT AMONGST THE PROTOZOA. 



The majority of Protozoa under certain conditions which are generally 

 adverse to their continued existence, or in anticipation of such conditions, 

 are able to enclose themselves in resistant capsules of varying degrees of 

 impermeability. Encystment is effected by the secretion from the surface 

 of the body of a substance which quickly hardens in the medium. In 

 the majority of instances, while this is taking place, the organism, which 

 has become contracted to a spherical form, revolves slowly, so that fresh 

 secreted material is applied regularly to the layer already formed. These 

 capsules are known as cysts, and are generally composed of a clear, hyaline, 

 transparent substance. In free-living Protozoa which live in water 

 encystment takes place when the medium is drying up, and there is danger 

 of desiccation. In this manner complete drying is prevented, and survival 

 for long periods may occur in conditions under which it would be im- 

 possible for the exposed organism to live. It has been shown that the 

 sand of the desert exposed to the tropical sun contains encysted Protozoa, 

 which emerge from their cysts when brought into more favourable sur- 

 roundings. The cysts are usually perfectly smooth on their outer surface, 

 but sometimes they are roughened by the formation of tubercles or other 

 markings. Very frequently, after the resistant cyst has been formed 

 there is secreted a membranous inner lining to the cyst. In such cases 

 one can distinguish a resistant r/>/r//.s7 from a more delicate cndocyst. 

 Sometimes cysts are pfovided with })ores, several of which are present 



