TH£ PHYLUM MOLLUSCA 



253 



Upper branchial 

 chambers 



^ t 



\ 



^ j^Lowcr 

 branchial 

 chambers 

 B C 



Figure 14.9. Diagrammatic cross sections showing difTcrcnl gill types in the I'clcry- 

 po(ia. ./. Oiilci I'liitdbiancliiala. (>ills slioil and siinpli'. It. Oiiici lilihranc liiata. (>ills 

 long and loldcd l>a(k. (.. Ordci l idatncllilnancliiata. I ike /•'. bill witli tlic folds fused 

 with Mian\ biidn's. 1). Order Septibranchiata. dills mo<lilied in Innn lioi i/iniial parti- 

 tions. (.After I ang.) 



creates water (itrreiits inward tluongli ilic iikiiikiu siphon, iijnvaid 

 throiigli many small sliis in the gills, and oiiiuaid ihioiigli the excur- 

 rent siphon. Ilic walci brings oxygen lor resjjiraiion at the gill sm- 

 faces, and many small lootl |jarti{les (algae and haderia) that are lia])|)C(l 

 on a miitons sheath setreted on the gills. Special tracts ol cilia move 

 this miK us toward the month, where it is eventually swallowed. I hits, 

 most pelecypods leetl by hltering water, and have little need loi loco- 

 motion. I he head is lediued to a mouth between a pair ol palps, long 

 folds ol (iliated skin dial colkct lood lioni ilie anterior edges ol the 

 gills and liaiislci ii lo the mouth. In this class ol moIliis(s the ladiila 

 is lacking. 



The i'elecyijoda are divided into orders (Fig. I !.'.>) according to the 

 detailed structuie ol the gilK. In the most primitive gioup the gills are 

 plumes in the |)osterioi pan ol the mantle cavity resembling those ol 

 the (hitons and snails. I lie |)alps are correspondingly enlarged to serve 

 in leeding. In othei gion|)s the gills are laige and lamellated with 

 numerous slits as desnibed above. Still other modihcations occur, but 

 most ol the lamiliar bivalves, such as mussels, clams, oysters and scallops, 

 ha\e the lamellatetl type. 



no. 



Venus mercenarla 



Ouahotf, liaidsliell tlam, littleneck and chcrrvstone are common 

 names lor I'cini.s incrcenaria (Fig. 1 1.10), a heart-shaped bivalve found on 

 the east coast of the United States. The young (cherrystones) are eaten 

 alive on liic hall-shell and the adults make excellent chowder. I'enxis 

 lives buiied head down in the sand anywhere Irom low tide to depths 

 of 100 leet, with the slujrt sijilioirs projecting to filter water. 



The shell valves are thick and strong. The hinge ligament, which 

 opens the valves, is antero-dorsal, next to the umbo, a prominent swell- 

 ing on each valve. .Anterior to the ligament are several prominent 

 teeth and several more apj^ear below the ligament as long ridges. These 

 dilfer on the two valves to form a rigid interlocking mechanism. The 



