XII 



PHYLUM MOLLUSCA 



681 



u 



am 



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 pallial 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 sinupalliale, 

 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. The siphons may be 

 separate (Fig. 589) or united 

 (Fig. 590). They are specially adapted for species of burro wing- 

 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 water and food, and even these can be instantly withdrawn in the 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 



u. 588. Venus gnidia, inner surface of 

 left valve, al, anterior lateral tooth ; am, 

 anterior adductor impression ; c, cardinal 

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

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

 terior adductor impression ; p. s. pallial 

 sinus ; . umbo. (From the Cambridge 

 Natural History.) 



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FH; Ys'.i Scrobicularia piperata, in it> natural position, partly buried in sand. A, 

 ' exhalant siphon ; II, inhalant siphon. (Kmm the Cambridge Natural Histoi " 



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 

 to form a sort of hood. 



To return to the shell, the muscular impressions and the pallia] 

 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 



