48 HOWARD AYRES. 
it seems not improbable that in such cases the spheridia may 
function as organs of light-perception—i.e. serving merely 
to recognise the presence of light; of course they can have 
not the least to do with sight. 
Theoretical.—The interesting and important fact, made 
known by the Hertwig brothers in their brilliant study of the 
nervous system of the Medusz, that there are frequently de- 
veloped in this class of animals, standing so low in the organic 
scale, sense organs of such specialised character that they 
appear out of all proportion with the degree of development of 
the nervous system which supplies them, is rendered doubly 
important when used in comparative anatomy, which discloses 
to us that among Ceelenterates these organs are frequently as 
highly developed as analogous organs among worms, molluscs, 
and tunicates. 
Among the Echinodermata we know of eye-spots among the 
Asteroids (and Echinids?), and of otolith sacs among the 
Holothurids (Synapta), but these organs never acquire the 
degree of specialisation seen in the otolith sacs and eye-spots of 
the Meduse. From Lovén’s description one would not be led 
to consider the spheridia of Echinids as so highly specialised 
as it appears we must now consider them. There is, in truth, 
a greater specialisation of parts, especially of the nerve-cells 
among these organs, than is to be seen in similar organs of the 
Medusz. Lovén has discussed the probabilities that the func- 
tion of these structures was either that of hearing, touch, taste, 
or smell, separately or combined in different ways (e.g. as organs 
with the combined function of taste and smell). The evidence 
at the present time is decidedly in favour of the view that they 
possess this double function. The following experiments serve 
to strengthen this view:—If one adds a drop of dilute acid acetic 
to the seawater containing the test of the Urchin under ob- 
servation it is easy to see the sudden stimulation and increased 
activity of all the external organs—spines, pedicellarie, 
spheridia, &c. The spheridia are the first to recognise the 
presence of the acid, and do so by one or two quick, short 
jerks, followed by aswaying or rotating motion. These motions 
