40 



PROTOZOA. 



cuticle as well as with the cilium. In a few forms (Actinobolus) 

 retractile tentacular processes are present. 



In the simplest forms the mouth is at the anterior end of the body, 

 the ciliary covering is uniform, and the cilia are nearly all the same 

 size (Enchelina). An advance upon this is presented by those forms 

 in which the mouth is on the ventral surface, and in which there is 

 a triangular area the peristome leading back from the front end of 

 the body to the mouth (Paramcecium). There may be a special row 

 of strong cilia, cirri, or menlbranellse the adored row, or zone 

 running along the left side of the peristome (Hypotricha, Fig. 30). 

 The peristome, when ventral, is very commonly asymmetrical. In 

 some cases the adoral row has the form of a spiral round the mouth 

 region, and the peristome may be placed at the front 

 end of the body (Stentor, Vorticella). In the Hypo- 

 tricha the following regions may be distinguished on 

 the ventral surface of the body : a pre-oral and a 

 post-oral region. The pre-oral region consists of the 

 peristome on the left side, and the frontal area on 

 the right (Fig. 41.) 



The majority of the Ciliata are multinuclear, and 

 the nuclei are almost, if not quite, always differ- 

 entiated into two kinds. One of these the macro- 

 nucleus is larger than the other, the micronucleus. 

 The macronucleus, which lies in the endoplasm, is 

 generally single, and presents considerable variation 

 of form; it may be spheroidal, band-shaped, 

 moniliform, or even branched. In some cases the 

 macronucleus is represented by a great number of 

 small nuclei. It is probable that these small nuclei 

 are all parts of one nucleus, and connected with one another by 

 fine fibres; so that the condition is really that of a much-branched 

 nucleus. Fragmentation of the macronucleus into fine pieces has 

 been observed, and is probably a stage in its final disappearance ; 

 but the real significance of the phenomenon is unknown. The 

 micronucleus is often multiple ; it varies in position, form, and 

 number, in different species. It is always smaller than the macro- 

 nucleus, and usually lies close to the latter. It appears only to be 

 absent in certain Opalines and in some of the so-called multinuclear 

 Ciliata just referred to. In the normal reproduction of the animal 

 it appears that, while the micronucleus divides by karyokinesis, the 

 macronucleus divides directly. 



FIG. 33 Opaliiia 

 ranarum (after 

 W. Engelmann). 



