i893. BIOLOGICAL THEORIES. 427 



in close relation with a large nerve ganglion, or rather pair of nerve 

 ganglia, with which the eyes and tentacles are also connected. The 

 organisation of the mollusc is, moreover, incomparably higher than 

 that of the jelly-fish, and it is not very rash to suppose that in this 

 case the shaking of the animal by the movement of the water, and 

 the consequent collision of the walls of the " otocyst " with the lithite 

 ("otolith") gives rise to a sensation. Whether it does so or not is, 

 however, of little moment. The ganglia are connected (indirectly) 

 with muscles, and the stimulus produced by the collision of lithite 

 and otocyst-wall may safely be assumed to lead to muscular move- 

 ments, either reflex or voluntary — movement which will take the 

 animal to safer zones of water. 



A shaking of the " auditory sac " of a crustacean would, by 

 jolting the " auditory" hairs against the lithites — mere sand particles 

 — produce a stimulus. The shaking might be produced in either of 

 two ways : it might arise from a tremor of the ground upon which, 

 or in which, the animal is resting, or from a disturbance of the water 

 about the iiagella of the antennule. Of the advantage of being thus 

 warned of the approach of an animal which either may serve as food 

 for the crustacean, or might feed upon it if it were taken unawares, 

 there need be no doubt. 



REFERENCES. 



1. Lamb, H. — " A Treatise on the Mathematical Theory of the Motion of Fluids." 



Cambridge, 1879. 



2. Basset, A. B. — " A Treatise on Hydrodynamics." Cambridge, 1888. 



3. Romanes, G. J. — " Jelly-Fish, Star-Fish, and Sea-Urchins." London, 18S5. 



C. Herbert Hurst. 



A CORRECTION :— 



In the previous article in this series (p. 352, 19 lines from the bottom), for 

 ■" 500 miles " read " 500,000 miles." 



