338 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
In the resting condition the valves of the shell are 
opened very considerably, but the organs in the pallial 
cavity cannot be seen owing to the fact that the edges of 
the upper and lower vela are just in contact. By putting 
a few grains of carmine in the sea water, an inhalent 
current can be demonstrated. This enters the pallial 
cavity by passing between the mantle lobes all round the * 
margin of the shell except for a small distance posteriorly. 
Here there is a strong exhalent current, and thus, 
although no morphological siphons are present, there are 
well-defined areas for the inhalent and exhalent respira- 
tory and nutritive currents. 
When the animal is about to swim, the following 
changes take place: the valves slowly open, that is, they 
move further apart than in the resting condition, and the 
visceral mass can be seen between the mantle edges. At 
the same iime the two vela he shghtly turned back 
against the mantle lobes as if moved inwards by the 
inflowing water due to the divarication of the valves. 
Towards the end of this opening motion the tentacles are 
quite suddenly retracted all round the mantle edge, and 
immediately the shell shuts with a snap. Just at this 
moment, however, the two vela take up the vertical 
position, with their margins touching, and by means of 
their muscular structure retain this position, acting as a 
perfect barrier to the water which must escape from the 
pallial cavity. The result is that the water escapes only 
where the two vela are not well developed, and where they 
do not dam back the current, and this is on each side of 
the dorsal edge of the shell. 
There are, therefore, two jets of water shot out 
dorsally at each sudden closing of the shell, for the process 
above described is repeated rapidly for several seconds, 
and consequently the animal moves onward with the 
