360 DIVISION IV. RADIATA.—CLASS V. INFUSORIA. 
when taken out of the water; others again are lobed or palmate, studded 
with prickly points, and perforated at intervals with oscula. These grow to 
a considerable size, but, like the former, are useless, since their substance is 
full of needles of flint.” 
CLASS V. INFUSORIA. 
Beyond the boundaries of visible animal existence the earth teems with 
forms of life of the most extraordinary character, and which occupy a most 
important place in the economy of nature. So small as to escape the scrutiny 
of the naked eye, and discovered only by the aid of the microscope, yet their 
calcareous shells, by immensity of numbers, and accumulating from century 
to century, form no small portion of the composition of the solid crust of 
the earth, and thus whole countries are built up of the remains of these 
microscopical creatures. 
As a general thing, the Infusoria, or Animalcules, have a gelatinous body, 
and a very simple organization. They constitute two orders. 
ORDER I. ROTIFERA. 
These creatures have an oval, gelatinous body, a mouth, a stomach, an 
intestine, and vent near the foot. They also have a tail variously formed, 
and on the fore part of the body an extraordinary organ, like one or more 
toothed wheels, which revolve with great rapidity. The genera are /uncu- 
laria, Trichocerca, Tubicolaria, and Branchionus. 
The Tubicolaria form little habitations for themselves, of foreign substan- 
ces, out of which they protrude the rotatory organs in a manner similar to 
the tentacula of Polypi. 
ORDER II. TWOMOGENEA. 
These have no viscera, or complex organization, and many of them have 
no mouth. They form a considerable number of genera, all of which 
resemble each other in the rapidity of their movements. Those constituting 
the genus Vibrio have a round body, like a very minute thread, and are 
often seen in vinegar. 
They move with great rapidity, darting hither and thither, in an apparently 
sportive mood, as we see minnows sport in their watery element. 
