MOLLUSCS. 



253 



istic features of these forms, — a shell divided into halves, one on either side of the 

 body. This bilateral symmetry pervades the whole organism, and frequently one 

 side is almost an exact repetition of the other. Just inside the shell is found the 

 fleshy mantle, which, like the shell it secretes, forms a flap on either side of the body. 

 In these bivalves this pallium, ov mantle, acquires a great development, and not infre- 

 quently its edges are joined together, so that the rest of the animal is enveloped, as it 

 were, in a bag. Still the bag is never completely closed ; at the front end a small 



Fig. 260. — Diagram of anatomy of a clam {Mya); a, anterior adductor; 6, auricle; c, exourreiit 

 siphonal tube; e, inourrent siplional tube; /, foot; g, gills; i, intestine; m, mouth; p, posterior 

 retractor; r, retractor of foot; t, labial palpi; v, ventricle of heart. 



hole is left for the protrusion of the foot, while, at the opposite extremity, means is 

 afforded for the entrance of water, bringing food and oxygen to the animal, and also 

 for the escape of the same fluid, bearing away the waste products of respiration and 

 digestion. Not infrequently this posterior opening becomes divided into two tubes, 

 which sometimes can be extended a long distance from the shell. This is known as 

 the siphon, and will readily be recognized by most people in the 'head' of the clam. 

 Head it certainly is not, for it is at exactly the opposite end of the body from where 

 the head should be. These tubes, which are, in reality, but expansions of the mantle, 

 are very contractile, and each tube has its own function. The lower one (the one fur- 

 thest from the hinge of the shell) is for the incurrent stream, while from the other 

 the water which has played its part in the economy of the animal is discharged. In 

 other forms there is no siphon, and in still others 

 the two halves of the mantle are entirely free 

 from each other. 



The mantle joins the body near the hinge 

 line, and between the two hang down the gills, 

 to which we shall again recur. From the lower 

 side of the body proceeds the foot, which in some 

 forms is well developed, even proving an organ 

 of locomotion of no mean capacities, while in 

 others, like the oyster, the foot has nearly, or even 

 entirely, disappeared. Near the extremity of the foot in the adults of some species, 

 and the young of others, is a gland, the function of which is to secrete the byssus. 

 This is a bundle of fibres, more or less closely united, by which the animal attaches 

 itself. The byssus can be cast off by the animal when desired, and a new one formed 

 at pleasure. ^ 



The mouth, which is at the opposite end of the body from that at which the siphon 



Fig. 261. — Mussel with byssal threads. 



