Chap. XIX.] VORTICELLA AND PARAMCECI 



V 



The animalcule which is to be encysted separates from its 

 contracts into a spheroidal body, around which a clear structure- 

 less envelope or cyst is formed. Within the cyst the endoplast 

 and the contractile vesicle, permanently expanded, are visible. 

 But ere long the contractile vesicle disappears, the band-like 

 endoplast increases much in size, and becomes convoluted. It 

 next becomes pinched in at intervals, so as to assume a beaded 

 appearance. And at last each bead becomes separately pinched 

 off from the rest. Thus the cyst appears full of bead-like spores. 

 Then the cyst bursts, and these spores are discharged as minute 

 oval or pear-shaped free-swimming animalcules, possessing an 

 anterior crown of cilia, in the midst of which there is a potential 

 mouth. These feed and multiply by fission, but ere long attach 

 themselves by the anterior oral end (E.). The girdle of cilia is 

 now lost; disc, peristome, and vestibule are developed, a 

 gradually lengthening contractile pedicle is acquired, and the 

 organism is once more a vorticella. This mode of reproduction 

 (the occurrence of which is strongly vouched for) does not seem 

 to be common among vortieellidse. More frequently encystment 

 is merely a temporary resting stage, during seasons of drought 

 perhaps, or other uncongenial conditions, from which the vorti- 

 cella emerges simply to resume its ordinary mode of life. 



Vorticella may be found adherent to the roots of duckweed. 



Paramoecium is found in pond-water or in vegetable infusions. 

 From its peculiar form (Fig. 102, F.) it is often known as the 

 slipper-animalcule. There is a thin superficial cuticle richly 

 supplied with cilia. Since in paramcecium and its allies the cilia 

 are more or less completely developed over the whole surface, 

 they are known as the Holotricha, as opposed to the Peritricha, 

 to which vorticella belongs, where there is a wreath of cilia, and 

 other groups in which the cilia are differently arranged. 

 Beneath the cuticle is a cortical layer, which is fibrillated in a 

 direction perpendicular to the surface. If dilute acetic acid bo 

 added to the water in which paramcecium is under observation, 

 the organism will become quiescent, and there will shoot out 

 from the surface a number of stiff processes. These accessory 

 structures are known as trichocysts. There is a bean-shaped 



