128 VORTICELLA AND ZOOTIIAMNIUM less. 



e', commcnceinenl of binary fission ; K-, coi)i|jletion of tlie process ; 

 e", the barrel-shaped product of division swimming freely in the 

 direction indicated by the arrow. 



F^ a specimen dividing into a megazooid and several microzooids (w) ; 

 ¥-, division into one mega- and one microzooid. 



g\ g^, two stages in conjugation showing the gradual absorption of 

 the microgamete (w) into the megagamete. 



Il', multiple fission of encysted form, the nucleus dividing into numer- 

 ous masses : H^ spore formed by multiple fission ; H-* — H^, development 

 of the spore ; u* is undergoing binary fission. 



(E— H after Saville Kent.) 



delicate cuticle (cu). There is a single contractile vacuole 

 {c. vac) the movements of which are very readily made out 

 owing to the ease with which the attached organism is kept 

 under observation. There is a meganucleus {fiu) remarkable 

 for its elongated band-like form, and having in its neighbour- 

 hood a small rounded micronucleus. Cilia are also present, 

 but the way in which they are disposed is very peculiar and 

 characteristic. To understand it we must study the form 

 of the body a little more closely. 



The conical body is attached by its apex or proximal end 

 to the stalk : its base or distal end is expanded so as to form 

 a thickened rim, the peristome {per), within which is a plate- 

 like body elevated on one side, called the disc (d) and 

 looking like the partly raised lid of a chalice. Between the 

 raised side of the disc and the peristome is a depression, the 

 mouth {mth), leading into a conical gullet {gull). 



There is reason for thinking that the whole proximal 

 region of Vorticella answers to the ventral surface of Para. 

 moecium, and its distal surface with the peristome and 

 disc to the dorsal surface of the free-swimming genus : the 

 mouth is to the left in both. 



A single row of cilia is disposed round the inner border 

 of the peristome, and continued on the one hand down the 

 gullet, and on the other round the elevated portion of the 



