200 



PROTOZOA. 



which a slender oesophagus, rarely supported by firm rods (Chilodon, 

 Nassulci), often projects (fig. 140). Through this the food stuff 

 passes into the endoplasm, in which it gives rise to food vacuoles. 

 The latter undergo a slow rotating movement round the body in 

 the endoplasm, which is caused by the contractility of the sarcode. 

 During this process the food is digested, and finally the solid, useless 

 remainder is ejected through the anal aperture. A digestive canal, 

 bounded by distinct walls, exists no more than do the numerous 

 stomachs which Ehrenberg, who was deceived by the food vacuoles, 

 ascribed to his Infusoria polygastrica. In all cases where a digestive 

 canal has been described, we have to do with peculiar strings and 

 trabeculae of the internal parenchyma which enclose in their inter- 

 stices spaces filled with a clear fluid. 



The more viscid exoplasm is pre-eminently 

 to be regarded as the motor and sensory layer 

 of the body. In it we find differentiations 

 resembling muscles (Stentor, the stalk of Vorti- 

 cell(t). Sometimes small rod-shaped bodies are 

 present (e.g., Bursaria leucas, Nassulci), which 

 are comparable to the thread cells of Turbellaria 

 and Ccelenterata. The contractile vacuoles appear 

 as further differentiations of the external layer, 

 structures which to the number of one or more 

 are found in quite definite portions of the body. 

 They are clear, mostly spherical spaces filled 

 with a fluid ; they diminish suddenly and then 

 vanish, but gradually reappear and increase to 

 their original size. These pulsating vacuoles 

 are usually connected with one or more vessel- 

 like lacume, which swell considerably during the contraction of 

 the vacuole. These structures have been compared to the water 

 vascular system of Botifera and Turbellaria, and have been explained 

 as excretory an interpretation which has in its favour the fact that 

 the contractile vacuoles in certain cases open to the exterior through 

 a fine pore at the surface, through which granules pass to the 

 exterior. 



The nucleus and nudeolus lie in the exoplasm of the infusorian 

 body. The nucleus, which ten years ago was compared to the nucleus 

 of the simple cell, is a structure of variable shape but with a definite 

 position in the body. One, or more than one, may be present. It 

 is sometimes round or oval, sometimes elongated, being drawn out 



FIG. 140. Chilodon citctd- 

 lus (after Stein), with 

 gullet resembling a 

 fish-basket. N, nucleus 

 with nucleolus, excreta 

 are passing out of the 

 anus. 



