VORTICELLA II9 



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 rhacronucleus and two 

 micronuclei. In a "pure line," all descended from one, there is no 

 conjugation. But there is a periodic, usually iflohthlry,: occurrence, 

 as Woodruff and Erdmann have shown, of a reriiarkable p/rocess called 

 endomixis. The nuclei break down as if thei;e.;was _goiDjg to be con- 

 jugation, and then there is re-organisation. ^V >!J- -'/^^ 



Fifth Type of Protozoa — 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 stenis 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. 



This contractile filament, under a high power, may exhibit 



a fine striation. (A similar striated structure is seen in 



some Amoebae, Gregarines, spermatozoa, etc., and of a much 



coarser type 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. On the side of this passage 



there is a weak spot, the potential anus, by which useless 



