INFUSORIA. 65 



from true 'vesicles' by the variations wliich con- 

 tinually occur in their size, number, and position. 



8. ;^'isc3oMs &v. — Most, ifnot all, of the77i/i(,- 

 soria are provided with one or more central solid 

 particles or ' nuclei,' the presence of which we 

 have already stated to be more or less charac- 

 teristic of the Protozoa. The nucleus varies in 

 position, being in most cases attached to some 

 part of the parenchyma of the body. It varies 

 also both in form and structure. Thus in Vorti- 

 cella and Stentor it is elongated, band-like, con- 

 sisting of an external membrane filled with granu- 

 lar contents, whilst in Ophryoglena (according to 

 Lieberkiihn) it is ovate, and destitute of any ap- 

 parent structure. In other Infusoria it may be 

 either round {Oxytricha), reniform {Loxodes), 

 shaped like a horse-shoe (Eitplotes), or spiral (as 

 in some species of Stentor). Sometimes, though 

 rarel}^, it is branched. In colour it is usually pale 

 yellow. 



In the o-ranular contents of some nuclei a cleai* 

 space or cavity is observable, within which a smaller 

 body termed the * nucleolus ' is placed. In other 

 cases it occurs on the exterior of the same orc^an. 

 Lieberkiihn describes the nucleolus of Ophryoglena 

 as minute, globular, structureless, and firmly at- 

 tached to the surface of the ovate nucleus. But 

 in Chilodon, the centre of the nucleolus is marked 

 by a transparent dot. 



A bright coloured particle (usually red), termed 

 the ^pigment-spot,' is found in the bodies of 

 many Infiisoria. In some it is altogether desti- 

 tute of structure, in others it is made up of a 

 number of exceedingly minute granules. 



