9 6 



MANUAL OF ZOOLOGY. 



the " cuticle " is the layer from which the cilia are given off, 

 and below that, again, is a finely striated or fibrillated contrac- 

 tile layer (" myophane layer " of Haeckel), which corresponds 

 physiologically to the muscles of higher animals. In some 

 Infusorians there is a still more internal lamina of the " cor- 

 tical layer," which is charged with the singular little organs 

 known as " trichocysts." These are vesicular microscopic 

 bodies, capable of emitting thread-like filaments, in many re- 

 spects closely resembling the " thread-cells " of the Ccelenterata. 

 The " cuticle " in Paramcetium is covered with vibratile cilia 

 (figs. 3 1 and 32), and is perforated by the aperture of the mouth. 



Fig. 32. A, Paramoecium^ showing the nucleus () and two contractile vesicles (v). 

 B, Paramoecium bursaria (after Stein) dividing transversely : n Nucleus ; n' Nucle- 

 olus ; v Contractile vesicle. C, Paramoecium aurelia (after Ehrenberg), dividing 

 longitudinally. 



The mouth leads into a funnel-shaped gullet, which is not 

 continued into a distinct digestive sac, but loses itself in the 

 soft central protoplasm. On the line of boundary between the 

 cortical layer and the diffluent central sarcode are placed the 

 "nucleus" and the "contractile vesicle" (or vesicles). The 

 " nucleus " is an oval body (in some forms band-shaped or rod- 

 like), consisting of an outer membrane enclosing granular con- 

 tents, and often having a smaller spherical particle applied to 

 its exterior or immersed in its substance. This latter is the 

 so-called "nucleolus," which must be carefully distinguished 

 from the nucleolus of a cell, which occurs in the interior of the 

 nucleus. The contractile vesicles are clear spaces, which con- 

 tract and dilate at intervals, and occasionally exhibit radiating 

 canals passing into the surrounding sarcode. Ordinarily one 

 contractile vesicle is present, or at most two, but in some cases 

 there may be several. It has also been maintained that the 



