192 ZOOLOGY 
edges of the remainder of the mantle are in contact. These 
openings form the dorsal and ventral siphons. In some 
Lamellibranchs, eg. Mactra, Cyclas, etc., these notches, by the 
fusion of their edges, are converted into tubes, which in some 
genera attain a length of several inches. The ventral siphon 
serves to admit fresh water, bringing with it oxygen and food, 
and the dorsal siphon gives exit to a stream of water which 
carries away the waste products and generative cells. 
The foot is not developed in the OSTREIDAE, and is 
small in Mytilus, the marine mussel. In the cockle, Cardiwm, 
and in 7rigonia, it can be suddenly bent, and by this means 
the animal jumps along. In Solen the foot is suddenly 
retracted, and in this way water is violently forced out of 
the siphons, and the animal is propelled forwards. Pecten 
flies through the water, with its dorsal surface downward, 
by the flapping of the valves of its shell. The foot often 
bears a special gland, which secretes a number of horny filaments 
known as the byssus, which serve to anchor the animal 
to the ground. This structure is well seen in Arca and 
in Mytilus. 
The mouth, which is median, and ventral to the anterior 
adductor muscle when the latter is present, lies in a 
groove formed by the anterior and posterior labial palps. 
These are ciliated structures, which resemble to some extent 
the gills, and doubtless serve to convey minute organisms 
to the mouth as food. The alimentary canal is ciliated. 
The stomach gives off a caecum, which in many 
genera lodges a crystalline style. The function of the style 
is obscure, but it appears to consist of an albuminoid 
material. The intestine is coiled, and leads to a straight 
rectum, around which the ventricle of the heart is often 
folded. A fold of the intestine, or typhlosole, increases 
its surface. A paired gland, the so-called liver, pours its 
secretion into the stomach. 
Two auricles return the arterialised blood to the ventricle, 
which in Arca is double; the ventricle gives off an anterior 
and posterior aorta, which distributes the blood all over the 
body. The blood from the mantle is in Anodonta returned 
directly to the auricles; the rest of the blood is collected into 
