444 LESLIE B. AREY AND W. J. CROZIER 
dorsal surface. In connection with these strange eyes and their 
supposed functional significance (Semper, '81; Bretnall, '19) a 
few observations and some suppositions have from time to time 
been recorded with reference to the behavior of Onchidium; 
but knowledge of its life and habits, which we find to exhibit 
some perplexing features of curious interest, has been strangely 
meager; nor has proof been offered that the 'eyes' are indeed 
photo-sensitive (cf. Crozier and Arey, '19c). 
At Bermuda, O. floridanum is quite generally distributed along 
the shores of the islands, commonly inhabiting protected loca- 
tions where the intertidal shore zone is covered by a layer of 
sun-bleached algae, although it is also found in intertidal habitats 
of other kinds (Crozier and Ai'ey, '19a). O. floridanum is an 
Antillean species closely resembling others of this genus found 
in the Pacific (Dall and Simpson, '01). Like most of its rela- 
tives, this species is strictly intertidal; some forms have been 
recorded as dwelling beneath low water, 
Onchidium is a good laboratory animal. It can be maintained 
in small dishes with a little sea-water for a month or longer, 
even if starved. In one case several individuals were kept in 
small (50-cc.) bottles, tightly stoppered, and half full of sea- 
water, in which diatoms had been planted. They were alive, 
active, and of normal appearance at the end of six weeks. The 
mollusks were observed to creep above the water in the bottles 
at irregular intervals, later returning under water and feeding 
upon the plant growths there. 
For experimental observations it is nevertheless desirable 
that freshly collected animals be employed, and this has been 
the rule in the work forming the basis of the following discussion. 
Our material came, in 1914-17, from Little Agar's Island, and 
in 1918 from the shore of Dyer Island, in each instance very 
close to the locations of the Biological Laboratory in these 
years. In addition, much of what we have learned concerning 
the behavior of Onchidium is the result of sun-baked vigils on 
the shore rocks in other parts of Bermuda. The work was 
initiated in 1914 by L. B. A., and has since then been extended 
by W. J. C. The observations here collected pretend to no 
