CHAPTER XII. 



THE WHEEL-BEARING ANIMALCULES. 



WE have now to consider another class of ani- 

 malcules. The Rotifera, or wheel-bearers, are 

 proved, by the recent discoveries of the microscope, 

 to be distinguished from the potygastrica by many 

 important differences of organization. They belong 

 to that division of the animal kingdom, termed, by 

 professor Owen, Nematoneura, from two Greek 

 words, meaning a thread and a nerve, from the 

 circumstance that it is here that the first traces of 

 distinct nerves, in the form of fine threads, begin to 

 make their appearance, and in some instances, 

 ganglia, or rudimentary nervous centres. In addition 

 to this character, distinct muscular fibres are per- 

 ceptible, arranged in bundles ; and, though there is 

 no heart or great receptacle for the circulating 

 fluid, there is a system of vessels for its due 

 distribution through the frame. The digestive 

 apparatus also assumes a more perfect state, and 

 instead of consisting of mere cavities in the sub- 

 stance of the body itself, it presents a true stomach 

 and intestinal canal, with various appendages. The 

 mode of reproduction is no longer by the division of 

 the parent or adult into new beings, but by eggs, 

 containing the vitalized germ of the future animals. 



