MOLLUSCA — PELECYPODS 



211 



tubes placed side by side and bound together. Through them 

 the blood circulates and their walls are bathed on all sides by 

 the water which pours into the gills between the tubes. As 

 these walls consist of a single layer of cells, there is a ready inter- 

 change between them of the waste gases (which the blood 

 brings from the body to the gills) and the oxygen of the water. 



Fig. 90. — The quahog, Venus mercenaria Linne, in position for feeding (natural 

 size), partly dissected lying in the right valve; a., anus; a.a., anterior adductor 

 muscle; a. r., anterior retractor muscle; e.5.,excurrent siphon; gi., cut edges of gills; 

 h., heart ; i.s., incurrent siphon ; /., ligament ; li., liver ; ma., mantle cut and pinned 

 back to show the organs within its cavity ; mo., mouth ; pa., palps or lips at side 

 of mouth ; p.a., posterior adductor muscle ; p.r., posterior retractor muscle. 



