20 
W. J. Dfikin 
The osphradium, termed formerly by the Germans »Geruchsorgan« 
(after Spengel), has often been considered an olfactory organ, without 
supporting evidence. It is at the same time referred to as an organ for 
the testing of the quality of the water entering the shell. An organ per- 
forming the latter function is I think, not necessarily an olfactory organ. 
The osphradium occurs always near or in close connection with the bran- 
chial nerve which innervates the respiratory organs, and this in animals 
which obtain their food in such different ways as the lamellibranchs andtho 
pectinibranchs. If the organ was an olfactory organ, it might be ex- 
pected that there would be some relation to the methodby which the animai 
obtains its food, instead of which there is apparently a relation to the 
respiratory mechanism, for according to Pelseneer (1906) it is absent 
in aerial species (of the gastropoda) or in aquatic forms devoid of a respi- 
ratory cavity. It may be possible that by means of this organ the quality 
of the water as regards respiration is determined. 
Freidenfelt (1897) states that his experiments show the organ 
to be rudimentary in the lamellibranchiata, but no clue to these experi-^ 
ments is given, and I do not believe the structure would be so well de- 
veloped as in Pecten, if rudimentary. 
Orientation. Pecten is always found resting on the same side^ 
the right, and an individuai turns over more or less quickly (according 
to its condition) if placed on the left side. 
The overturning is caused like the swimming movements by a violent 
expulsion of water from the palliai cavity. This is controlled by the 
velum which, contrary to its usuai action in swimming, allows the water 
to escape ventrally, and, using the hinge line as a fulcrum, the animai 
turns itself over. 
The immediate result of placing a specimen on its left side, is the 
divarication of the two valves (after the sudden closure caused by the 
disturbance) which move further apart than is normaUy the case, sa 
that the gills and viscera may be seen, between the mantle folds. The 
tentacles of the mantle edge are extended and move from side to side 
whilst this opening movement is continued. FinaUy, after a somewhat 
more rapid opening still, the tentacles are retracted and the adductor 
muscle contracts violently forcing the water out ventrally and turning 
the animai over. It is quite evident that the mantle edge, the velum, 
and the adductor muscle are under special control and their movements 
are coordinated, those of the velum and adductor being almost auto-^ 
matic. This is illustrated still further by Stimulation of the visceral 
ganglion. A weak current of electricity was used, and was applied by 
