MOLLUSCA PROPER. 197 
when they are very long, they are furnished with muscles to 
retract them within the shell, and it is the scar left by these 
muscles which causes the pallial line to be indented. This 
indentation, therefore, as seen in the dead shell, is an indi- 
cation that the animal possessed long retractile respiratory 
siphons, and lived, therefore, most probably embedded in sand 
or mud. 
There is always a distinct heart, composed of two or three 
chambers, and in all cases acting as a mere arterial heart. 
That is to say, the heart propels the aerated blood derived from 
the gills through the body, and has nothing to do with the pro- 
pulsion of the non-aerated or venous blood through the gills. 
There is never any distinct head in any of the Bivalves, and for 
this reason they are sometimes called the “headless” (acepha- 
Jous) Molluscs. The mouth is simply placed at the anterior end 
of the body, and is never furnished with teeth, though usually 
_ provided with membranous processes or “ palpi” (fig. 100, p). 
The mouth opens into a gullet which conducts to a stomach. 
The intestine is convoluted, and usually perforates the ventricle 
of the heart, ultimately terminating in a distinct anus, which is 
always placed near the respiratory aperture. A large and well- 
developed liver is also present. 
The nervous system has its normal form of three principal 
masses—the cerebral, the pedal, and the parieto-splanchnic 
ganglia. 
The majority of the bivalve Molluscs have the sexes distinct, 
but they are sometimes united in the same individual. The 
young are hatched before they leave the parent, and, when first 
liberated, are ciliated and free-swimming. 
The habits of the Lamellibranchiata are very various. Some, 
such as the Scallops (Pecten), habitually lie on one side, the 
lower valve being the deepest, and the foot rudimentary or 
wanting. Others are fixed to the bottom of the sea by the 
substance of one of the valves. Others, such as the com- 
mon Mussel, are moored to some foreign object by a tuft of 
silky fibres, constituting a “byssus.” Many, such as the 
Gapers (Aya) and Razor-shells (So/ez), spend their existence 
sunk in the sand of the sea-shore or the mud of estuaries. 
Others, such as the Pfolades, bore holes in rock or wood, 
in which they live. Finally, many are permanently free and 
locomotive. 
