p^'t. 



Fig. 72. — / 'orticclla. 

 \, living specimi::!) fully expanded, showing stalk {st) with axial fibre {ax.f)^ 

 peristome {pi^r)-, disc {d), mouth {inth), gullet {gull), and contractile vacuole. 



]j, the same, bent 011 its stalk and with the disc turned away from the observer. 



C, optical section of the same, showing cuticle (cu)^ cortex (co?-t), medulla {nwd), 



^ nucleus («7^), gullet (^/c//), se\'eral food-vacuoles, and anus (rt«), as well as the 

 structures shown in A. 



Ul, a half-retracted and D- a fully-retracted specimen, shou'iug the coiling of the 

 stalk and overlapping of the disc by the peristome. 



K^, commencement of binary fission; E^, completion of the process; E'', the 

 barrel-shaped product of division swimming freely in the direction indicated 

 by the arrow. 



Fl, a specimen dividing into a megazooid and several microzooids (m) ; F-, 

 division into one mega- and one microzooid. 



Ql, G-, two stages in conjugation showing the gradual absorption of the micro- 

 gamete {m) into the megagamete. 



H^- multiple fission of encysted form, the nucleus dividing into numerous masses ; 

 H-, spore formed by multiple fission ; H^ — H*", development of the spore ; H^ is 

 undergoing binary fission. (From Parker's Biology : E — H after SaviUe Kent. 



