1891.] Mechanical Origin of Structure in Pelecypods. 17 
be observed that the borders of the shell gape at either end. This 
is evidently caused by the constant extension of the foot and 
siphons, which, pressing on the mantle border, thus keep it back 
at those areas and modify the direction of shell deposition. 
Another active cause for the gaping of the valves is doubtless 
the loss of the habit of withdrawing the organs and closing the 
valves as a source of protection. Such forms as we are considering 
are protected in a measure by the surrounding sand or other 
medium, and in time of danger seek safety by burrowing deeply. 
In Mya arenaria we find a highly elongated siphon. In the 
young the siphon hardly extends beyond the borders of the 
valves, and then the animal lives at or close to the surface. In pro- 
gressive growth, as the animal burrows deeper, the siphon elon- 
gates, until it attains a length many times the total length of the 
valves. The ontogeny of the individual and the paleontology of 
the family both show that Mya came from a form with a very 
abbreviated siphon, and it seems evident that the long siphon of 
this genus was brought about by the effort to reach the surface, 
induced by the habit of deep burial. 
In the structure of Pecten irradians we find the most complete 
adaptation to the mechanical requirements of the act of swim- 
ming. Pecten swims by the rapid opening and closing of the 
valves, with the resultant violent expulsion of water; but the 
details of the method are somewhat intricate. In swimming, as 
well as when at rest, the left valve is always uppermost, and the 
plane of the edges of the valves is inclined to the surface of the 
water at an angle of about 45°. The mantle folds are built up 
in perpendicular walls on the periphery, and these walls perform 
an important function in swimming. Lying on the bottom, the 
Pecten suddenly closes its valves by the quick action of the 
adductor muscle. The first water expelled is driven out posteri- 
orly in the direction of the arrow a, Fig. 5, and if this were the 
only or the main direction in which a current is expelled, the 
animal would by impact of water be driven in the opposite direc- 
tion, or anteriorly, which is not the case. When the valves have 
closed to a slight extent, the borders of the two thick perpendic- 
ular mantle walls come in contact, and then no more water is 
