NATURAL HISTORY OF ONCHIDIUM 457 
the shadow of an approaching fish induced the discharge from 
the glands of a Hquid spray which drove off the fish. This notion 
was extended by Semper so as to provide a mechanism explain- 
ing the development of the mantle-eyes of Onchidium. He 
considered that the distribution of those species of Onchidium 
possessing mantle-eyes coincided with that of the intertidal 
blenny Periopthalmus. This remarkable fish skips along the 
beach zone laid bare by the falling tide (cf. figure in Hess, '12), 
and was said by Semper to prey upon Onchidium. The testi- 
mony of later naturalists does not favor this view. Periopthalmus 
lives upon mud flats and frequents the margins of mangrove 
swamps (Murray, p. 489; Eliot, '99), whereas Onchidium lives 
upon rocks and along the edges of reefs, where the blenny is not 
found (Eliot, '99). Periopthalmus, moreover, feeds on arthropods 
(Murray, loc. cit.). Australian species of Onchidium possess 
mantle-eyes, and in regions where no Periopthalmus-like fishes 
are known (Bretnall, 1919). Although Onchidium is quite sen- 
sitive to shading (Crozier and Arey, '19c), 0. floridanum does 
not discharge its poison glands when stimulated by a sudden 
decrease of light intensity; tactile excitation is the form of stim- 
ulation pre-eminently successful in eliciting discharge of the 
glands. The dorsal surface of 0. floridanum is, however, the 
part sensitive to shading, while one of the most conspicuous 
features of the animal's response to shading is the retraction of 
the tentacles, the latter organs being, nevertheless, themselves 
devoid of direct excitability by shading. 
Neither Periopthalmus nor any other fishes of analogous habit 
occur at Bermuda. It is improbable that fishes are able to enter 
the majority of the Onchidium nests, because of the minuteness 
of the entrances. It seems likely that it is in air, rather than 
when under water, that the glands are most effective. The gland 
contents are discharged in a stream which in air breaks up into 
a fine, almost invisible, spray, which may be thrown as far as 
15 cm., or about ten times the length of the Onchidium. Under 
water, long threads of secretion are expelled, which do not form 
a spray and fail to travel more than a centimeter or so from 
the apertures of the glands. Tactile stimulation of the dorsal 
