214 .MORPHOLOGY OF INVERTEBRATE TYPES 



Respiratory system. Venus mercenaria has two pairs of 

 gills or ctenidia. However, the two gills of the same side must 

 be regarded morphologically as a single gill. They are out- 

 growths or folds of the body wall, attached to the body on each 

 side along an almost straight line extending from the anterior 

 end of the liver almost to the lower end of the adductor of the 

 valves. The inner gill is broader than the outer one and both 

 have a striated or ridged appearance. Each gill consists of two 

 lamellae joined at regular intervals by parallel connections called 

 interfoliary bridges. These bridges correspond to the grooves 

 between the ridges or folia of the gill and divide the space inside 

 the gill into water tubes. These tubes are lined with ciliated 

 epithelium. They open dorsally into the upper branchial cham- 

 ber of the gill which communicates directly with the cloacal 

 siphon. The folia or ridges of the gill are covered with an im- 

 mense number of small, ciliated filaments. The gills are natu- 

 rally rich in blood-vessels, the largest among which run chiefly 

 in the interfoliary bridges. Water is drawn into the mantle 

 cavity through the incurrent or branchial siphon by the com- 

 bined action of the ciliated epithelium of mantle and gills, passes 

 into the water tubes through the incurrent openings of the gill, 

 oxidizes the blood, collects in the upper branchial chamber and 

 is ejected through the cloacal siphon. 



Nervous system. The nervous system of Venus mer- 

 cenaria is typical not only of all lamellibranchs but in a way 

 also of other molluscs. Although devoid of a head separate 

 from the rest of the body, the lamellibranchs naturally possess 

 a central nervous system corresponding to that in other mol- 

 luscs. It consists of three paired ganglia. Of these the pair of 

 cerebropleural ganglia is situated above the oesophagus imme- 

 diately under the anterior retractor of the foot. The pair of 

 pedal ganglia is imbedded in the tissues of the foot close under 

 the place where the stomach opens into the intestine. Finally 

 the pair of visceroparictal ganglia is situated on the anterior 

 surface of the posterior adductor of the valves. The cerebro- 



