PHYLUM MOLLUSCA 



699 



these circumstances the posterior pallial muscles become en- 

 larged to form retractors of the siphons, and the portion of the 

 pallial line from which they arise becomes, as it were, pushed 

 forwards so as to form a bay or pallial sinus (Fig. 578, p.s). Thus 

 the shells of species with well- 

 developed siphons are sinupal- 

 liate, or have an indented 

 pallial line, while those with 

 small or no siphons are in- 

 tcyripalliate, or have an entire 

 pallial line. The larger the 

 siphons the stronger are their 

 muscles and the deeper is the 

 pallial sinus : when very large 

 they cannot be completely 

 retracted, and the posterior 

 border of the shell then gapes 

 permanently. The siphons may 

 be separate (Fig. 579) or 

 united (Fig. 580). They are 

 specially adapted for species 

 of burrowing habits, which are 

 able to remain buried in the 

 mud or sand, only the ends of 



the siphons being exposed for the supply of aerated water and 

 food, and even these can be instantly withdrawn in the event of 

 danger. 



In addition to their union posteriorly to form the siphons, the 

 mantle-lobes may concresce to a greater or less extent along their 

 ventral border (Fig. 581), forming a more or less tubular invest- 

 ment for the body, and leaving an anterior pedal aperture for the 



FIG. 578. Venus gnidia, inner surface of 

 left valve, al, anterior lateral tooth; am, 

 anterior adductor impression ; c, cardinal 

 teeth ; /. ligament ; lu. lunule ; p, pallial 

 line ; p. I, posterior lateral tooth ; p. m, pos- 

 terior adductor impression ; p. s. pallial 

 sinus ; u. umbo. (From the Cambridge 

 Natural History.) 



Fi... 07'.'. Scrobicularia piper ata, in its natural position, partly buried in sand. A, exhalant 

 siphon ; B, inhalant siphon. (From the Cambridge Natural History.) 



protrusion of the foot. Their anterior portions may also be united 

 to form a sort of hood. 



To return to the shell, the muscular impressions and the pallial 

 line on which have already been referred to. As a general rule 

 the right and left valves are alike, or nearly so, the shell being 

 therefore equivalve. Each valve is inequilateral, being divided into 



Y Y 2 



