XII 



PHYLUM MOLLUSCA 



681 



aperture or short tube. In the Sinupalliata the two siphons are 



prolonged into distinct muscular tubes (Fig. 587, A, B) which, in 



the position of extension, project beyond the posterior margin of the 



shell and may even be considerably longer than the body. Under 



these circumstances the posterior pallia! muscles become enlarged 



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



line from which they arise is, 



as it were, pushed forwards 



so as to form a bay or pallial 



sinus (Fig. 588, p.s). Thus 



the shells of species with well- 



developed siphons are sinupalliate, 



or have an indented pallial line, 



while those with small or no 



siphons are integripalliate, 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 re- 



tracted, and the posterior border 



of the shell then gapes perman- 



ently. Ihe Siphons may be 



separate (Fig. 589) or united 

 (Fig. 590). 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 water and food, and even these can be instantly withdrawn in the 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 



FIG. 588. Venus gnidia, inner surface of 

 left valve, al, anterior lateral tooth ; am, 

 anterior adductor impression ; c, cardinal 

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



Cambridge 



sinus ; u. umbo. (From 



FIG. 589. Scrobicularia piperata, in its natural position, partly buried in sand. Am, 

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



ventral border (Fig. 591), forming a more or less tubular invest- 

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

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

 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 



VOL. I XX 



