484 THE SENSORIAL FUNCTIONS. 



exhibits apparently the lowest and most riidimental 

 condition of the animal functions; and it is diffi- 

 cult to conceive how so simple an organization can 

 exercise those of sensation and perception. Yet 

 the actions of the Hydra, of which I have given an 

 account, are indicative of distinct volitions ; as are 

 also, in a still more decided manner, those of the 

 Infusoria. In the way in which the latter avoid 

 obstacles while swimming in the fluid, and turn 

 aside when they encounter one another, and in the 

 eagerness with which they pursue their prey, we 

 can hardly fail to recognise the evidence of volun- 

 tary action. 



To seek for an elucidation of these mysteries in 

 the structure of animals whose minuteness precludes 

 all accurate examination, would be a hopeless 

 inquiry. Yet the indefatigable Ehrenberg has 

 discovered, in some of the larger species of ani- 

 malcules belonging to the order Rotatoria, an 

 organization, which he believes to be a nervous 

 system.* He observed, in the Hydatina senta, a 

 series of six or seven grey bodies, enveloping the 

 upper or dorsal part of the oesophagus, closely 

 connected together, and perfectly distinguishable, 

 by their peculiar tint, from the viscera and the 

 surrounding parts. The uppermost of these bodies, 

 which he considers as a ganglion, is much larger 

 than the others, and gives off slender nerves, which, 

 by joining another ganglion, situated under the 

 integuments at the back of the neck, form a circle 

 of nerves, analogous to that which surrounds the 

 oesophagus in the mollusca : from this circle two 



Vnnales des Sc. Nat. serie 2, iii. 



369. 



