VORT2CELLA. 93 



The details of the conjugating process have been worked out with 

 great care by Maupas and others. They differ slightly in different 

 species ; what occurs in P. aurelia is summarised diagrammatically in 

 Fig. 22. 



The micronuclear elements are represented by two minute bodies. 

 As conjugation begins, these separate themselves from the macronucleus. 

 The macronucleus degenerates, and each micronucleus increases in 

 size (A). Each divides into two (B) ; another division raises their num- 

 ber to eight (C) ; seven of these seem to be absorbed and disappear, the 

 remaining eighth divides again into what may be called the male and 

 female elements (D) ; for mutual fertilisation now occurs (E). After 

 this exchange has been accomplished, the Infusorians separate, and 

 nuclear reconstruction begins. The fertilised micronucleus divides into 

 two (F), and each half divides again (G), so that there are four in each 

 cell. Two of these form the macronuclei of the two daughter cells 

 into which the Infusorian proceeds to divide (H) ; the other two form 

 the micronuclei, but before another division occurs each has again 

 divided. Thus each daughter-cell contains a macronucleus and two 

 micronuclei. 



Fifth Type VORTICELLA. 



Vorticella, or the bell animalcule, is a type of those ciliated 

 Infusorians in which the cilia are restricted to a region 

 round the mouth (Peritricha). 



Groups of Vorticella, or of the compound form Carchesium, 

 grow on the stems of fresh water plants, and sometimes are 

 readily visible to the unaided eye as white fringes. In 

 Vorticella each individual suggests an inverted bell with a 

 long flexible handle. The base of the stalk is moored to 

 the water weed, the bell swings in the water, now jerking 

 out to the full length of its tether, and again cowering down 

 with the stalk contracted into a close and delicate spiral. 

 In Carchesium the stalk is branched, and each branch 

 terminates in a bell. Up the stalk there runs, in a slightly 

 wavy curve, a contractile filament, which, in shortening, 

 gives the non-contractile sheath a spiral form. This con- 

 tractile filament, under a high power, may exhibit a fine stria- 

 tion. (A similar striated structure is seen in some Amoebae, 

 Gregarines, spermatozoa, &c., and above all, in striped 

 muscle fibres. It seems to be some structural adaptation 

 to contractility.) The bell has a thickened margin, and 

 within this lies a disc-like lid ; in a depression on the left 

 side, between the margin and the disc, there is an opening, 

 the mouth, which leads by a distinct passage into the cell. 



