458 LESLIE B. AREY AND W. J. CROZIER 
surface of the mantle is the kind of excitation most effective for 
gland discharge. Among the animals noted as frequenting the 
Onchidium zone were included: isopods (Ligia, especially), 
crabs (Sesarma, Panopeus, Porcellana, in some places Pachy- 
grapsus, and an occasional Portunus), Chiton tuberculatus, 
Coscinasterias (during its breeding season — January to February — 
often left above low water, in a depression between rock slabs 
and in similar places), and the mud-dauber wasps (Polistes), 
which gather moist silt from cool, shaded, intertidal spots near 
the entrances to caves. Unequivocal instances have been 'found 
in which crabs, isopods, starfish, and wasps came into contact 
with Onchidium; in most, if not indeed in each of these instances 
there occurred immediate moderate discharge of the glands, 
followed by the retreat of the animal from the Onchidium. It 
must not be supposed that these creatures were endeavoring to 
devour the Onchidia; rather, it seemed important for the snails 
to avoid being accidentally pushed off the rock into the water, for, 
as previously noted, Onchidium does not adhere with any great 
firmness to the algae-covered rock. When purposely pushed 
off, into the water, an Onchidium is not able to return to its 
nest. It is entirely probable that most of the creatures acci- 
dentally touching one of these snails would have retreated even 
in the absence of the repugnatorial secretion, but the importance 
of the discharge is nevertheless clear. 
The gland secretion was obtained in a 'pure' state by holding 
a glass slip over the back of an Onchidium stimulated mildly, 
in air, by means of faradic shocks. The glands individually 
turn their apertures dorsalward and their axes converge in such 
fashion that the several discharges impinge very nearly at a 
single point immediately above the site of stimulation. This 
conspicuous accuracy involved in release of the gland content is 
a noteworthy feature of the use of the glands. Small bits of 
crab- and mussel-meat were smeared with the secretion and 
were found to be vigorously rejected as food by sea-anemones, 
star-fish, crabs, and fishes, including forms which could not by 
any possibility have previously encountered this material, as, for 
example, Fundulus from landlocked brackish ponds at Ber- 
