144 BIOLOGY OF THE PROTOZOA 



podia, filopodia and lobopodia being expressions of energy transfor- 

 mations comparable with the rotation of protoplasm in Xitella or 

 circnlation in Tradescantia. Axopodia of the motile Heliozoa, 

 axial filaments of the inactive species, and stereoplasmic cores of 

 the rhizopodia, may be regarded as successive phases in the modi- 

 fication of vibratile flagella. These types of pseudopodia have in 

 common an enveloping layer of granular protoplasm, but filopodia 

 and lobopodia represent a different t,\pe, being made up in large 

 part, or entirely, of ectoplasm and without any evidence whatsoever 

 of kinetic elements. So-called "contractile elements" of this type 

 of pseudopodia are largely figments of the imagination. 



3. Cilia.— Cilia are the motile organs of Infusoria and accompany 

 the most highly differentiated types of cortex to be found in the 

 Protozoa. Individually they are shorter, more -delicate and less 

 powerful than flagella and owe their importance as motile organs 

 to their large numbers and s;>'nchronous beating. Their action 

 may be compared with that of oars in rowing, while flagellum action 

 might be compared with sculling, and the residts of cilia and flagella 

 activities bear a relation similar to that between a racing shell and 

 a gondola. 



According to the interpretation of several observers, mainly 

 Schuberg, Maier, Schubotz, Schroder, etc., the cortex of ciliates is a 

 composite of zones of differentiated protoplasm. In the majority of 

 cases such zones cannot be made out, for one shades into the other, 

 and the whole into the alveolar endoplasm. In favorable cases, 

 however, we can distinguish (1) a superficial periplast perforated 

 for the exit of cilia and trichocysts when present; (2) an alveolar 

 layer containing trichocysts if the latter are present; (3) a contrac- 

 tile zone containing the basal bodies of cilia, myonemes and coordin- 

 ating fibers; (4) a denser zone which shades off into the endoplasm 

 and supplies an anchorage for nuclei and contractile vacuoles. 



A single cilium is constructed on much the same plan as a flagel- 

 lum, consisting of a central axial filament or fiber, and an elastic 

 sheath of protoplasm. Movement is due to the active contraction 

 in one plane of the axial fiber and recovery to the elasticity of the 

 enveloping sheath. The contractile element originates from a basal 

 body in the contractile zone. In Pycnoikrix monocystoides, accord- 

 ing to Schubotz (1908), there are two basal bodies, one internal, 

 which are connected by a rhizoplast. Connecting these basal 

 bodies is a series of longitudinal and transverse coordinating fibrils 

 (Fig. 54, p. 105). 



The arrangement of cilia on the surface of the body varies in 

 different species; sometimes they form a complete coating for the 

 organism as in the majority of Holotrichida (Fig. 7()) ; sometimes 

 they are limited to certain zones as in Urocenfrum turbo, Didinium 

 nasvfvni, etc. (Fig. 168, p. 383); or sometimes to the ventral surface, 



