265 
When all the muscles of the mantle are relaxed, and the foot is 
protruded nearly to its full extent, the anterior opening is much larger 
than the foot, and through it, water can freely enter the mantle 
chamber. 
When the foot is partially withdrawn and the sphincter muscle 
around the opening through which it protrudes contracts, the mantle 
is brought in close contact with the foot, and this channel is com- 
pletely obliterated. 
Let us now consider how the animal forces such strong jets of 
water through the posterior opening of the mantle chamber. 
Beginning with the animal in the condition in which it has just 
been described, with the anterior opening closed, a forcible contraction 
of both the longitudinal and radial pallial muscles takes place, accom- 
panied by a further retraction of the foot. In this way the fused 
margins of the mantle and the elastic cuticle, are forced in between 
margins of the calcareous portion of the shell. At the same time, 
the adductor muscles contract and the valves are brought together so 
that the calcareous margins are as close together as may be without 
injuring the tissue between them. 
Fig. 11. Fig. 12. 
Fig. 11. Transverse section of Solenomya velum, taken a little anterior to the 
posterior adductor muscle, where the mantle chamber, when expanded, is large. The 
mantle chamber is here shown expanded; c cuticle of the shell; s position occupied by 
the calcareous portion of the shell; m mantle. 
Fig. 12. A similar section of Solenomya velum, showing the mantle and shell 
cuticle folded into the mantle chamber, 
The effect of this folding of the mantle and flexible cuticle is in- 
dicated by the two transverse sections, Figs. 11 and 12, taken through 
the posterior portion of an animal, where the mantle chamber, when 
