NATURAL HISTORY OF ONCHIDIUM 489 
DISCUSSION 
1. It is desirable to deal, as briefly as may be, with certain of 
the more general implications of the conclusions provided by our 
inquiry into the habits of Onchidium. 
The Onchidiidse are a group well calculated to cause the zoolo- 
gist trouble. For a long time the taxonomic affinities of these 
organisms were hazy and in dispute, for it was bj^ some (Bergh, 
'95; Fujita, '97) supposed that, in addition to dermal respiration 
accomplished through mantle papillae (conspicuously developed 
in certain species) when under water, air breathing was also 
carried out, but by the organ regarded as a kidney — an idea once 
used as the foundation of von Ihring's class 'Nephropneusten,' 
but now known to have resulted from an imperfect acquaintance 
with the difficult morphology of the true lung (Plate, '94; von 
Wissel, '98; Pelseneer, '01). Thus we are probably dealing with 
a member of a typical land group, Pulmonata, which has second- 
arily taken up the habit of living on the seashore. It would be 
valuable to know whether Onchidella is a more archaic type than 
Onchidium proper (Plate, '94), or the reverse. According to 
Bretnall ('19), Onchidium damelii lives either altogether below 
low water or between tidal hmits. 
2. The activities of these animals are not less curious than 
their presumptive evolutional history. In the case of numerous 
invertebrates of the shore zone it has seemed possible to provide 
a clear description of behavior in terms directly stated by the 
outcome of analytical experunents. In fact, this general method 
of study has been made the basis of much recently published 
work in animal ecology. The ethology of Chiton (Arey and 
Crozier, '19) and of Chromodoris (Crozier and Ai'ey, '19 b} 
can be followed with gratifying completeness from relatively 
simple experimental results. With Onchidium the situation is 
more subtly complicated, and for the purpose of ecologic inter- 
pretation the isolated results of such a method are here ahnost 
meaningless, as shown conspicuously by the creature's heho- 
tropic responses. Michael ('16) and others have recently been 
to some trouble to emphasize the fairly obvious point that no 
