92 PROTOZOA THE SIMPLEST ANIMALS. 



division into two (Fig. 37, div.). One-half includes the 

 "mouth," the other has to make one. As well as this 

 simple fission, a process of transient conjunction also occurs. 

 Two individuals approach one another closely, the two 

 nuclei of each break up, an exchange of pieces of the 

 micronucleus takes place; the two then separate, each to 

 reconstruct its two nuclei (Fig. 38). This process is neces- 

 sary for the continued health of the species. 



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. 39. 



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 

 number 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). 



Description. — Groups of Vorticella, or of the compound 

 form Carchesium, grow on the stems of fresh-water plants, 

 and are sometimes 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. 



