NERVOUS SYSTEM OP INVERTEBRATA. 379 



the intervention of any common sensorium, or centre of ac- 

 tion. It is at the same time remarkable that their move- 

 ments are not effected by means of muscular fibres, as they 

 are in all other animals, the granular flesh, of which their 

 whole body is composed, appearing to have a generally dif- 

 fused irritability, and perhaps also some degree of sensibi- 

 lity; so that each isolated granule may be supposed to be 

 endowed with these combined properties, performing, inde- 

 pendently of the other granules, the functions both of nerve 

 and muscle. Such a mode of existence exhibits apparently 

 the lowest and most rudimental condition of the animal 

 functions. 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 swim- 

 ming 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 struc- 

 ture of animals whose minuteness precludes all accurate ex- 

 amination, would be a hopeless inquiry. Yet the indefati- 

 gable Ehrenberg has recently discovered, in some of the 

 larger species of animalcules belonging to the order Roti- 

 fer u y an organization, which he believes to be a nervous sys- 

 tem. He observed, in the Hydatina senta, a series of six 

 or seven gray 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 slender nervous filaments are sent off to 

 the head, and a larger branch to the abdominal surface of 



