WHEEL-BEARERS. 
199 
take the form of slender but lengthened threads, and others 
are seen, crossing, in various directions, like a net, the 
ampler viscera. From this rich muscular development, the 
movements of the Rotifer a are very varied and vigorous. 
The nervous system is very peculiar and remarkable in 
this class. It exists in a form wholly without parallel in 
the Invertebrate Classes, namely, that of a single mass, well 
defined, and often of immense comparative size, which, in 
spite of all our reluctance, wo can compare only to the 
brain of the Vertebkata. This great nerve-mass is placed 
in the head of the animal, and commonly carries, seated 
on its hinder part, a red eye (sometimes two) of simple 
structure, apparently composed of a lens and a pigment 
mass. The great ganglion sends off a pair of threads to 
the nape, w r here they unite into a small ganglion below 
an organ of sense, which, in its most ordinary form, is a 
tubular projection, furnished with a terminal tuft of hairs, 
and which appears to be the representative of the antenna; 
in Insects. In a few cases we have dotected threads of 
extreme tenacity floating in the cavity of the body, which 
we believe to have a nervous character ; but besides these, 
we know of no offshoots from the great central brain. It 
is certain that nothing answerable to the double chain of 
ganglia, united by threads, and running down the length 
of the body, that ai'e so characteristic of the Annulosa 
generally, exists in this class. 
Scarcely less anomalous is the condition of the mouth. 
Far down in the body is seen an oval pellucid mass, con- 
taining a curious array of symmetrical pieces that work 
vigorously on a central table, like a pair of hammers on an 
anvil. Strange as is its position, this organ is nothing 
