MOLLUSCA : LAMELLIBRANCHIATA. 



325 



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 Sinupallialia (fig. 131, 2). 



The nervous system of the Lamellibranchiata is composed of 

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

 parieto-splanchnic or branchial. The so-called "organ of 

 Bojanus " of the bivalves is doubtless mainly concerned in 

 excretion, and in all probability represents the kidney. There 

 is one of these organs on each side of the body, each com- 

 posed of two sacs separated from those of the opposite side 

 by a venous sinus. Or it may be looked upon as a double 

 organ composed of two bilaterally symmetrical halves. It is 

 situated just below the " pericardium," and communicates with 



Fig. 131. Shells of Lamellibranchiata. i. 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. 



it and also with the mantle-cavity. Though undoubtedly per- 

 forming the functions of a kidney, the organ of Bojanus is also 

 connected kisome cases with reproduction, and it appears to 

 correspond to the " pseudo-hearts " of the Brachiopoda. 



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 

 liberated, 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 mus- 

 cles which close the shell and those which form the " foot." 

 (fig. i3o,/). The "foot" is present in the majority of bivalves, 

 though it is not such a striking feature as in the Gasteropoda. 



