6 



111 the resting condition the valves of the shell are 

 opened very considerably, but the organs in the pallial 

 cavity cannot be seen owing to ilie 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 time the two vela lie slightly 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 Avhich 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 



