192 ZOOLOGY 



edges of the remainder of the mantle are in contact. These 

 openings form the dorsal and ventral siphons. In some 

 Lamellibranchs, e.g. 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 Osteeidae, and is 

 small in Mytilvs, the marine mussel. In the cockle, Cardium, 

 and in Trigonia, 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 fllaments 

 known as the lyssus, which serve to anchor the animal 

 to the ground. This structure is well seen in Area 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 Area 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 



