CHAPTEE XIII. 



UNICELLULAR ANIMALS (PROTOZOA) {Continued). 



B. Infusoria. 



{Paramoecium, Vorticella, etc.) 



Infusoria are minute unicellular animals found like Amoeba 

 in stagnant water or in organic infusions (see p. 201) (hence 

 "Infusoria"). In the leading features of their organization 

 they are closely similar to Amoeba and its allies, from which 

 they differ, however, in having a much higher degree of differ- 

 entiation, in mo^dng by means of cilia instead of j)seudopodia, 

 and in showing the first indication of gamogenesis (amphimixis). 

 Paramoeciimi (the slipper-animalcule) is an actively free- 

 swimming form often found in multitudes in hay-infusion or 

 water containing the decomposing remains of Nitella and other 

 water-plants. Yorticella (the "bell-animalcule ") is commonly 

 attached by a slender stalk to duck-weed (Lemnoi) and other 

 water-plants, or to other submerged objects ; at other times it 

 breaks loose from the stalk and swims for a while actively about. 

 The two forms are constructed upon essentially the same plan, 

 but Yorticella shows in some respects a much higher degree of 

 differentiation. 



Paramoecium. — The slipper-shaped body (Fig. 87) is covered 

 with cilia by means of which the animal rapidly swims about. 

 Morphologically the body is a single cell, having the same gen- 

 eral composition as in Amoeba^ but possessing in addition a deli- 

 cate surrounding membrane ("cuticle") or cell-wall. The 

 differentiation of the protoplasm into ectoplasm and entoplasm 

 is very sharply marked, and the former contains numerous 

 peculiar rod-like bodies {trichoGysts) from which long threads may 

 be thrown out. Their function is probably that of offence and 

 protection. As in Amoeba the protoplasm contains water-vacu- 



oles (w.v) and food-vacuoles (f.v) (both of which are carried 



168 



