MOLLUSCA : LAMELLIBRANCHIATA. 



401 



fresh water, whilst the effete water is expelled by the other. 

 The margins of these " inhalant " and " exhalant " apertures 

 are often drawn out and extended into long muscular tubes 

 or " siphons," which may be either free, or may be united to 

 one another along one side (fig. 208, s s), and which can usually 

 be partially or entirely retracted within the shell by means of 

 special muscles, called the " retractor muscles of the siphons." 

 These siphons are more especially characteristic of those La- 

 mellibranchs which spend their existence buried in the sand, 

 protruding 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 deter- 

 mined 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 un- 

 broken in its curvature, and presents no indentation (Integro- 

 pallialid). 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 Sinupallialia (fig. 210, 2). 



The nervous system of the Lamellibranchiata is composed 



Fig. 210. 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. 



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

 parieto - splanchnic or branchial. The principal organs of 

 sense are the tactile labial palpi, otocysts, and eye-spots. The 

 otocysts are not always present, and the ocelli, when present, 

 are almost always placed round the edge of the mantle. 



The so-called " organ of Bojanus " of the bivalves is doubt- 

 less mainly concerned in excretion, and in all probability re- 



2 C 



