n PHYLUM PROTOZOA 93 



peristome. They contract so as to produce a wave-like movement 

 which aids in the ingestion of food. In some cases (Fig. 71, 11) 

 the undulating membrane (n, nib.) is a very large and obvious 

 structure. 



Certain peculiar forms have yet to be mentioned. Multicilia (Fig. 

 71, 12) has an irregular body of varying form, and bears a small 

 number of very long flagellum-like cilia. Another genus in which 

 the cilia approach to flagella is LnpTiomonas (13), the ovoid body of 

 which bears a tuft of close-set cilia at its anterior end. Actino- 

 bolus (14) is remarkable for the possession, in addition to cilia, of 

 long retractile tentacles used for attachment. In Didinium ( 15) 

 the barrel-shaped body is encircled by two hoops of cilia. 



As we have seen, the meganucleus in Paramcecium is ovoid : in 

 other genera it may be elongated and band-like (3, m-g. nu.), horse- 

 shoe-shaped (9), very long and constricted at intervals so as to 

 look like a string of beads (16), or much convoluted and branched 

 (17). In some genera the meganucleus undergoes repeated 

 divison, forming at last a very great number of small bodies only 

 discoverable by staining : this process of fragmentation of the nucleus 

 may proceed so far that the protoplasm of a stained specimen has 

 the appearance of being strewn with granules of chromatin. The 

 discovery of f this phenomenon has tended to throw doubt on the 

 reported total absence of a nucleus in some Rhizopods. 



In nearly all species one or more micronuclei are present, the 

 number sometimes reaching nearly thirty. In Opalina (Fig. 75) 

 numerous nuclear bodies (nu.) are present, some of which on 

 account of their mitotic mode of division are to be regarded as 

 micronuclei, while the rest are meganuclei. 



In Vorticella and other peritrichous genera there is a single 

 contractile vacuole (Fig. 73, c. vac), which, like that of Euglena, 

 opens through the intermediation of a reservoir into the vestibule. 

 In the remaining Ciliata there may be one, two, or many some- 

 times a hundred contractile vacuoles. They may be scattered 

 all over the cortex (Fig. 71, 18), or arranged in one or two rows 

 (8). The star-like arrangement of radiating canals, described in 

 Paramcecium, occurs in several genera : or there may be two long 

 canals, or the number of these channels in the protoplasm may 

 reach thirty (19, c). In some instances the protoplasm is hollowed 

 out by numerous non-contractile vacuoles (18, vac.) so as to 

 have a reticulate appearance, reminding us of the extra-capsular 

 protoplasm of Radiolaria. 



Trichocysts, like those of Paramcecium, are found in many 

 holotrichous forms, but arc rarely present in the other subdivisions 

 of the order. In the peritrichous Epistylis umbellaria, however, 

 there are found numerous minute capsules (Fig. 71, 9, ntc.) 

 arranged in pairs, each containing a coiled thread. They are 



