NERVOUS SYSTEM OF INVERTEBRATA. 539 



a still more decided manner, those of the Infu- 

 soria. In the way in which the latter avoid ob- 

 stacles 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 

 voluntary 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 Ehren- 

 berg has recently discovered, in some of the 

 larger species of animalcules belonging to the 

 order Rot if era, an organization, which he be- 

 lieves 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 con- 

 siders as a ganglion, is much larger than the 

 others, and gives off slender nerves, Avhich, by 

 joining another ganglion, situated under the in- 

 teguments 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 sur- 

 face of the body. The discovery of a regular 



