PKOTOZOA. 



79 



lias no contractility ; but it is extensible and elastic, and is thus 

 able to follow the contractions and expansions of the protoplasm. 



In addition to this separation of the 

 cuticular layer there is also, in the Gre- / 



garinas, a cortical layer separated off from 

 the internal parts, which appears to be 

 finer than the richly-granulated protoplasm ; 

 this is also found in the Infusoria. 



Fig. 24.-. GregariuEe from 

 the enteric canal of Opatrum 

 sabulosum ; a is the younger 

 stage, provided with a " pro- 

 boscidiform" process, a An- 

 terior portion, b Posterior 

 portion of the body, c Nu- 

 cleus. 



§ 62. 



With the separation of the body into an 

 external cortical layer, and an internal par- 

 enchymatous substance, further metamor- 

 phoses of the cortical layer are connected. 

 Of these the first that should be mentioned 

 are the cilia which are widely distributed 

 in the Infusoria. They appear to be direct 

 but actively motile prolongations of the in- 

 tegument : if combined with a cuticle they 

 traverse it. They either beset a limited part 

 of the body only, as the so-called oral open- 

 ing*, or they are extended over larger tracts, or over the whole body 

 often, with great regularity. 



According to the definite distribution and arrangement of these 

 cilia, the Infusoria have been subdivided into Holotricha, Hetero- 

 tricha, Hypotricha, and Peritricha. It is clear that they are differ- 

 entiations of the protoplasm, from what happens in some other 

 groups of the Protista, where they form temporary structures only, 

 and, like the pseudopodia, can be again withdrawn into the proto- 

 plasm of the rest of the body. 



The flagella, as well as the undulating membranes, which are 

 often formed in the region of the mouth of many Infusoria, are 

 modifications of the cilia. The cilia appear sometimes to be modified 

 in a peculiar way to form stiff processes, movable only at their point 

 of attachment to the body (Stylonychia); sometimes, indeed, they 

 are flattened and broadened. 



The cilia, as well as the style-shaped processes, serve as locomotor 

 organs, and show us that locomotion is connected with the integu- 

 ment, just as it was connected with the temporary external layer of 

 the body, where pseudopodia were formed. 



Another structure observed in the cutis of many Infusoria (e.g. 

 Paramecium), are firm rod-like bodies (Trichocysts), which, under 

 certain influences, emit a fine stiff filament. These structures lie 

 close to one another in the cortical layer, and in a direction per- 

 pendicular to the long axis of the body. They call to mind the 

 stinging cells of the Coelenterata, but they are not to be regarded as 

 the same things, for they are not formed from cells. 



