254 



MANUAL OF ZOOLOGY. 



siphons are more especially characteristic of those Lamelli- 

 branchs which spend their existence buried in the sand, pro- 

 truding their respiratory tubes in order to obtain water, and 

 with it such nutrient particles as the water may contain. 

 The presence or absence of retractile siphons can be readily 

 determined merely by inspection of the dead shell. In those 

 bivalves in which siphons are not present, or if present are 

 not retractile, the ' pallial line ' in the interior of the shell is 

 unbroken in its curvature, and presents no indentation (Inte- 

 gro-pallialia). In those, on the other hand, in which retractile 

 siphons exist, the pallial line does not run in an unbroken 

 curve, but is deflected inwards posteriorly, so as to form an 

 indentation, or bay, which is termed the ' pallial sinus,' or 

 ' siphonal impression,' and is caused by the insertion of the 

 retractor-muscle of the siphon. Those bivalves in which this 

 sinus exists form the section Sinu-paJMalia (fig. 76). 



a 



a 



Fig. 76. Shells of Lamellibranchiata. 1. Cyclas amnica, a dimyary shell with 

 an entire pallial line. 2. Tapes pullastra, a dimyary shell with an indented 

 pallial line. 3. Perna ephippium, a monomyary shell. (After Woodward.) 

 a. Pallial line ; b. Muscular impressions left by the adductors ; c. Siphonal 

 impression. 



The nervous system of the Lamettibranchiata is composed 

 of the three normal ganglia the cephalic, the pedal, and the 

 parieto- splanchnic, or branchial. 



The majority of the bivalves are dioecious, but in some the 

 sexes are united in the same individual. The young are 

 hatched before they leave the parent, and are, when first libe- 

 rated, ciliated and free-swimming. 



The muscular system of the Lamellibranchs is well deve- 

 loped. Besides the muscular margin of the mantle, and the 

 muscles of the siphons (when these exist), there are also pre- 

 sent other muscles, of which the most important are the 

 muscles which close the shell, and those which form the 'foot.' 

 The ' foot ' is present in the majority of bivalves, though it is 

 not such a striking feature as in the Gasteropoda. It is essen- 

 tially a muscular organ, developed upon the ventral surface of 



