INFUSORY ANIMALCULES. 139 



133. The Animalcules to which the name of INFUSORIA 

 may be properly restricted (the jRotifera, or Wheel- Animal- 

 cules, 105, whose organization is much higher, together with 

 many organisms whose true nature is vegetable, being ex- 

 cluded), present an advance upon the simplicity of the Khizo- 

 poda in this, that whilst their bodies consist for the most 

 part of sarcode, and present scarcely anything that can be 

 termed a distinction of organs, their external surface is con- 

 densed into a membrane too firm to admit either of indefinite 

 extension into pseudopodia, or of the passage of alimentary 

 particles through it ; and consequently the form of the body, 

 although not insusceptible of being temporarily changed by 

 pressure, possesses a considerable degree of constancy for each 

 species (fig. 80). A mouth, or definite aperture for the in- 

 gestion of food, is provided; with an additional orifice in 

 some instances, through which indigestible or effete matters 

 may be discharged from the interior. Into this mouth, ali- 



Fig. 80. INFUSORY ANIMALCULES. 



i. Monads ; n. Trachelis anas ; in. Enchelis, discharging faecal matter; iv. Para- 

 nuEcium; v. Kolpoda; vi. Trachelis fasciolaris. 



mentary particles are drawn by the agency of the cilia with 

 which some part of the surface of the body is provided; 

 these cilia being always so disposed as to serve at the same 

 time for the general locomotion of the animalcule, and for the 

 production of currents that shall bring food to its interior. 



134. Although most Infusoria move freely through the 

 water in which they live, yet certain kinds of them attach 

 themselves by footstalks to marine plants or other floating 

 bodies, during at least a part of their lives ; and in this con- 

 dition bear no slight resemblance to Zoophytes, though of far 

 simpler organization. It is in these sessile forms that the 

 agency of the cilia in creating currents which bring food to 



