466 LESLIE B. AREY AND W. J. CROZIER 
Animals from which the tentacles had been removed had the right 
or the left half of the dorsal surface smeared with a mixture of lamp- 
black and vaseline. When a narrow beam of svmlight was allowed to 
fall on the blackened side of the body, no response of any kind was 
observed. When the unpainted side was exposed to the light, how- 
ever, these animals behaved like normal ones — they oriented precisely 
and always toward the darker side. 
The distribution of photic irritability, which thus seems to be 
confined to the dorsal surface of the mantle, is sufficient to show 
physiologically that differentiated receptors are concerned in 
the reactions of Onchidium to light. The exclusive photic 
irritability of the dorsal surface of the body is important in 
connection wdth the \vell-known mantle-eyes present in some 
members of this genus. Two views have been advanced con- 
cerning the phylogeny of these eyes: (1) that under bionomic 
stress the eyes have developed from some less specialized integu- 
mentary photoreceptors, this being the essence of Semper's 
idea, and (2) that eyes of this type were early developed by the 
primitive Onchidium stock, and have subsequently become in 
some species lost or rudimentary (cf. Stantschinsky, '08). In 
O. floridanum there are no differentiated mantle-eyes, and, 
although tentacular eyes are present, no evidence has been 
forthcoming to show that they actually play a part in the 
creature's activities, or indeed that they are in any degree photo- 
sensitive. A physiological analysis of the photic sensitivity of 
Onchidia possessing undoubted mantle-eyes should afford some 
data valuable in this connection, but has yet to be made. 
When the light falling upon an Onchidium quietly creeping 
in air is suddenly decreased, the tentacles of the. moUusk are 
quickly and forcibly withdrawn beneath the mantle, the head is 
retracted, locomotion stops, and the mantle is lowered into con- 
tact with the substratum. In the case of a snail which has for 
some time been undisturbed in this way, a very, slight decrease 
in light intensity induces response of almost maximum ampli- 
tude. In any event, the snail quickly resumes the attitude it 
had previous to stimulation, even though the reduced illumina- 
tion be maintained. If a shadow be cast on the anterior end 
only, the head is sharply withdrawm, locomotion ceases, the 
