PEOTOZOA: INFUSORIA. 59 



CHAPTER VI. 



INFUSORIA, 



THE Infusoria of many writers comprise many of the lowest 

 forms of plants such as the Diatoms together with the 

 Rotifera, a class of minute animals now known to belong to the 

 Annuloida. By modern writers, however, the term Infusoria 

 is used strictly to designate those Protozoa which possess a 

 mouth and rudimentary digestive cavity. They are, for this 

 reason, often called collectively the ' stomatode ' Protozoa in 

 contradistinction to the remaining members of the sub-king- 

 dom, which are all 'astomatous.' The name Infusoria itself 

 is derived from the fact that the members of the class are 

 often developed in organic infusions. 



The Infusoria, or Stomatode Protozoa, may be denned as Pro- 

 tozoa which are provided with a mouth and rudimentary digestive 

 cavity, which do not possess the power of emitting pseudopodia, 

 but which are furnished with vibratile cilia, or with contractile 

 filaments. They are mostly microscopic in size, and their bodies 

 usually consist of three distinct layers. 



The Infusoria may be divided into three orders, viz. : Suc- 

 toria, Ciliata, and Flagellata, of which the second comprises the 

 majority of the members of the class, and alone requires much 

 consideration. 



I. ORDER CILIATA. This order comprises those Infusoria 

 in which the outer layer of the body is more or less abundantly 

 furnished with vibratile cilia, which serve either for locomotion or 

 for the procuring of food. As types of the order Parammciwni 

 and Vorticella may be selected, the former being free, whilst 

 the latter is permanently fixed in its adult condition. 



Paramcecium (fig. 10 c) is a slipper-shaped animalcule, 

 composed externally of a structureless transparent pellicle 

 the ' cuticle,' which is lined by a layer of firm and consistent 

 sarcode, which is termed the ' cortical layer,' or the * paren- 

 chyma of the body,' this in turn passing into a central mass 

 of softer and more diffluent sarcode, known as the 'chyme- 

 mass,' or ' abdominal cavity.' The ' cuticle ' is covered with 

 vibratile cilia, and is perforated by the aperture of the mouth. 

 The mouth leads into a funnel-shaped gullet, which is not 

 continued into any distinct digestive sac, but is lost in the 

 central 'chyme-mass.' Within the 'cortical layer' are the 

 ' nucleus ' and ' nucleolus,' and the ' contractile vesicle ' (or 

 vesicles). The nucleus is a solid band or rod-shaped body, 



