STRUCTURE AND FUNCTION OF THE SPHARIDIA. 49 
are continued until the removal of the acidulated water or until 
the death of the Urchin. Chromic acid acts much the same as 
acetic acid. There are two possible explanations of these 
phenomena: either the spheridia have for their function the 
perception of such chemical changes in the surrounding water 
(t.e. taste and smell), and the reporting of the same to the 
nervous centres of the animal, from whence the inteiligence is 
sent out to the spines and pedicellariz, which latter are at once 
alert to secure the food-substance, whatever it may be (é. e. in 
the normal conditions of the animal’s life), or the spheridia 
are not organs with such function, and are merely more sensi- 
tive than the spines and pedicellariz, which are themselves 
capable of detecting chemical changes, though in less degree 
than the spheridia. The first one seems to me the true expla- 
nation, especially when the following experiments are taken 
into consideration. 
Sounds, whether loud or low, do not seem to affect the 
spheeridia in the least; but if the water containing the Urchin 
be thrown in vibration—as, for example, by striking the glass 
vessel with some steel instrument a light, sharp blow—the 
sphzridia are not set in motion, and appeared not to recognise 
the vibrations ; but the spines and pedicellariz are immediately 
affected, and begin swaying motions; the spines on that side 
of the Urchin facing the point of the vessel struck direct them- 
selves towards this point. 
VOL. XXVI.—NEW SER. D 
