NATURAL HISTORY OF ONCHIDIUM 459 
muda (Crozier, '19 b). When received upon one's tongue the 
repugnatorial spray is found to sting Hke wild mustard, and with 
considerable persistence (Crozier and Arey, '19 a). 
It is not Ukety that many creatures able to inflict damage 
upon Onchidia can gain access to them while they are concealed 
within a 'nest' and covered by the sea. Among those which 
need be mentioned in this connection one of the most interesting 
is the curious littoral chilopod Hydroschendyla.^ These rare 
forms occur at Bermuda between tides, in crevices and within 
the muddy interstices of much-eroded sandstone blocks. Like 
their geophilid relatives, they devour annelids, for they have 
been uncovered in the act of biting into the sides of Leodocid 
worms which occur between the aeolean strata.*^ On several 
occasions Hydroschendyla has been obtained within Onchid- 
ium nests when these were chiseled open. No indication was 
had, however, of either symbiotic or predatory connection 
between these forms — their association seemed entirely accidental. 
SENSORY RESPONSES 
1 . Mechanical excitation 
The responses of Onchidium to tactile excitation are of some 
diversity, depending upon the part activated. The surface of 
the foot responds by attachment when brought into contact 
with a surface sufficiently large; the glands upon the periphery 
of the mantle become erected and release their contents when 
the mantle is stroked or pinched, but otherwise the reactions to 
touch are of the more or less local and negative (withdrawing) 
type. General mechanical activation by water currents induce 
negative rheotropism. A kind of anemotropism, involving the 
^ We are indebted to Dr. R. V. Chamberlin, Museum of Comparative Zoology, 
for the identification of this chilopod. (Cf. Chamberlin, 1920.) 
^ Geophilus has been figured wrapped spirally about the body of an earth- 
worm which it had begun to devour. Hydroschendyla, however, seems merely 
to bite into the body of Leodocids, whereupon the worms conveniently auto- 
tomize at that place, the anterior end creeping away while the centipede sucks 
the juices of the abandoned tail. It is interesting to observe that allied Schen- 
dylids are known to frequent caves (Ribaut, '15). 
