254 ^'^f ANIMAL KINGDOM 



Umbo 



Teeth 



A\ tachmanl of 

 ant euMuctor 



Ligament 



Allachmenl oF 



Palp 



post, adductor 



Ant. addiictor 

 Attachme-nt 

 oF mantle 



sl.adductor 



Siphons 



Mantle 



Foot 



B 



■Ventricle 



Stomach 



Esophagi 

 Digestive gland' 

 Palp (lifted to 

 show mouth) 



Intesti 

 Crystcilline style.' 



icle 

 testine 

 rogznita.! papilla 

 Upper branchial chamber 



•Anus 



•E:>cCarrent: siphon 



InCurrcnt Siphon 



inner^ill ia 

 branchial chamber 



Gonad 



Figure 14.10. Anatomy of Venus merceimria. A, Interior of the right valve. B, Left 

 side with shell and mantle removed. C, Partial dissection, showing some of the internal 

 organs. 



valves can be shut by the adductor muscles so completely that Venus 

 will live for days out of water, a convenience for shipping them inland. 

 Live, healthy specimens can be purchased in almost any seafood market. 



The foot can be extended some distance out of the shell, and is 

 usually thrust anteriorly (downward) into the sand as an anchor. Venus 

 can also move slowly by movements of the foot. 



As in many clams, each of the four gills is attached along the dorsal 

 limit of the mantle cavity, hangs down toward the ventral limit, and 

 then folds back dorsally (Fig. 14.9 C). Each inner gill is attached to the 

 base of the foot, while each outer gill attaches to the mantle, forming a 

 total of four longitudinal upper gill chambers that come together 

 posteriorly at the excurrent siphon. Bridges of tissue between the folds 

 of each gill keep the folds slightly apart and create channels leading 

 from the many tiny slits in each fokl up to the upper gill chambers. 

 W^hile I'enus is feeding the shells are slightly open and the siphons ex- 

 tended. Water passes in the lower siphon, through the several gill folds 

 into the upper branchial chambers, and out through the upper siphon. 

 On the free surface of each gill fold the mucous sheath is swept ven- 

 trally to the lower edge of the fold, and then forward to the palps. If 

 accepted, the mucus and food are carried up the folds of the palps into 

 the mouth. Sometimes, however, dirt or distasteful material may enter 

 the mantle cavity, collect on the mucous sheath, and arrive at the palps 

 as though it were food. This is rejected, and transferred to the mantle 

 where the cilia move it ventrally and then posteriorly to accumulate 

 just below the incurrent siphon. Periodically it is ejected through the 



