MOLLUSCA. LAMELLIBRANCHIATA. 129 



s, s') ; these are sometimes free from one another, some- 

 times united, and they may be as much as four times the 

 length of the shell. The 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 In many forms the siphons can be withdrawn into the 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 two valves of 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 

 Dimyaria ; others, known as the Monomyaria, possess one 

 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 form the anterior muscle is found to be 

 present in the young state. 



The mouth (o) is placed in the middle line of 

 the body, ventral to the anterior adductor muscle, 

 and it is not provided with organs of mastication or 

 prehension. At its sides, are two leaf-like processes, 

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

 oesophagus, which passes into a globular stomach sur- 

 rounded by the liver ; then comes the intestine, which 

 after undergoing many convolutions, reaches the dorsal 

 surface of the body, where it passes through the peri- 

 cardium and is surrounded by the ventricle of the heart, 

 and afterwards ends in an anus (v) situated dorsally to the 

 posterior adductor muscle. In the lamellibranchs there 

 is no head, hence the class is sometimes spoken of, as the 

 Acephala. The nervous system consists of three pairs of 

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

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

 w. p. 9 



