INFUSOEY ANIMALCULES. 
139 
133. The Animalcules to which the name of Infusoria 
may be properly restricted (the Eotifera , 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 Bhizo- 
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- 
i. Monads; n. Trachelis anas; in. Enchelis, discharging fsecal matter, iv. Para- 
mcecium; 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 
