WHEEL-BEARERS. 



190 



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 Rotifera 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, we can compare only to the 

 brain of the Vertebrata. 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, where 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 antennae 

 in Insects. In a few cases we have detected 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 are 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 



