66 MANUAL OF ZOOLOGY. 



CHAPTER VI. 



INFUSORIA. 



THE Infusoria of many writers comprise many of the lowest 

 forms of plants such as the Diatoms together with the Roti- 

 fera, 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-kingdom, 

 which are all "astomatous." The so-called "suctorial" Infu- 

 soria (Acinetce\ however, appear to have no definite oral aper- 

 ture; and the same is the case with the parasitic Opalina, 

 though there is great doubt as to propriety of placing this in 

 the Infusoria at all. 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 defined as Pro- 

 tozoa which are mostly provided with a mouth and rudimentary 

 digestive cavity, which do not possess the power of emitting pseu- 

 dopodia, 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. : 

 Suctoria, 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. Besides cilia, properly so called, 

 some of the ciliated Infusoria are provided with styles or jointed 

 bristles which are movable and subserve locomotion; whilst 

 others have little hooks or uncini, with which they can attach 

 themselves to foreign bodies. As types of the order, Paramce- 

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

 whilst the latter is permanently fixed in its adult condition. 



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

 posed externally of a structureless transparent pellicle the 

 " cuticle," which is lined by a layer of firm and consistent sar- 

 code, which is termed the " cortical layer," or the " parenchyma 

 of the body," this in turn passing into a central mass of softer 



