200 MOLLUSCA. LAMELLIBRANCHIA 



ventral is generally the longer ; it is furnished with tactile 

 papillae, and is known as the branchial siphon (s), the 

 dorsal being the anal siphon (s). In many forms the 

 siphons can be withdrawn into the shell by means of 

 muscles. Occasionally, as in Teredo, the siphons are 

 surrounded by a calcareous tube. 



The shell can be closed by means of the adductor 

 muscles (a, a!), which pass from the interior of one valve 

 to the other. In many genera there are two adductors, 

 and these forms are frequently spoken of as the Dimyaria; 

 others, known as the Monomyaria, possess one adductor 

 only, and when this is the case it is the posterior which 

 is present, the anterior having atrophied ; this occurs in 

 the oyster, but in this, and in all other forms so far as is 

 known, the anterior muscle is present in the young state. 



In the lamellibranchs there is no head, hence the class 

 is sometimes spoken of as the Acephala. The mouth (o) 

 is placed in the middle line of the body, ventral to the 

 anterior adductor muscle, and is not provided with organs 

 of mastication. At each side are two leaf-like processes, 

 the labial palps (t). The mouth leads into a short oeso- 

 phagus, which passes into a globular stomach surrounded 

 by the liver; next is the intestine, which, after under- 

 going many convolutions, reaches the dorsal surface of 

 the body, where it passes through the pericardium and 

 is surrounded by the ventricle of the heart. The anus 

 (v) is situated dorsally to the posterior adductor muscle. 

 The nervous system usually consists of three pairs of 

 ganglia. One pair is placed at the sides of the mouth and 

 is connected by nerve-cords with a pair in the foot, and 

 with a third pair placed beneath the posterior adductor 

 muscle. From these ganglia nerves are given off to the 

 muscles, gills, etc. Tactile organs are present on the 



