NATURAL HISTORY OF ONCHIDIUM 445 
exhaustiveness; they do, however, throw valuable light upon 
some vexed questions, particularly in connection with the theory 
of adaptation. It is therefore important to remark that upon 
some of the peculiar features in this account of the behavior of 
Onchidium our respective observations, secured in an entirely 
independent manner, have been found in essential agreement at 
every point of overlapping. Elsewhere we have already com- 
mented on certain points in the behavior of Onchidium (Arey 
and Crozier, '18; Crozier and Arey, '19a, 1919 c). It is our 
purpose here to bring together in a unifying way the results of 
our previous communications, presenting fully the evidence 
concerned and indicating its place in a more systematic account 
of the natural history of this animal. 
OCCURRENCE; HOMING ACTIVITIES; COLORATION; 
REPUGNATORIAL GLANDS 
If, during the period of low water, on a sunny day, one inspect 
the shore rocks at Bermuda laid bare by the tide, at a place 
protected from the dashing of the surf, one frequently has little 
difficulty in discovering numbers of small blue-black slug-like 
Onchidia, about 15 mm. long or smaller. They feed quietly 
upon the thin coating of felted algae or creep about in a manner 
which at first sight seems aimless. Favorable localities are found 
on the lee shores of smaller islands within the semi-enclosed 
sounds (Great Sound, Castle Harbor, but not in Harrington 
Sound, where there is practically no tidal rise and fall and where 
no Onchidia occur). The animals also live, however, in bays on 
the north and south shores of Bermuda, but not where they 
would be du'ectly exposed to the ocean surf. 
The body of the adult Onchidium when resting undisturbed 
is dome-like, oval in outline, at most 17 mm. long X 12 mm. 
broad X 6 mm. high. At the anterior end, two slender tentacles, 
with knobbed ends, project from under the mantle-fold. Two 
large 'oral lappets,' the 'cephalic lobes' of Pelseneer ('01, p. 20), 
overlie the mouth region; in creeping they are constantly in slight 
contact wdth the substratum. The margin of the encircling 
mantle-fold is serrated; certain of the marginal projections rep- 
