336 



MOLLUSCA. 



TS 



VR 



chief of which are two lateral sinuses placed at the bases of the 

 gills and a pedal sinus leading into the large median sinus or 

 vena cava in the floor of the pericardium. The course of the 

 circulation, though it cannot be certainly determined, seems to be 

 somewhat as follows : the blood from the mantle, which acts as 

 a respiratory organ, returns directly to the heart; of the rest of 

 the systemic blood, part is supposed to go direct to the gills, and 

 thence to the auricles ; the bulk of it, however, is probably collected 

 into the vena cava, whence it pastes through blood spaces in the 



adjacent kidneys to the gills, and 

 thence to the heart. There is a valve 

 at the junction of the main pedal 

 sinus with the vena cava, which closes 

 during the turgescence of the foot. 



The blood is generally colourless, though 

 in some forms it has a bluish tint owing 

 to the presence of haemocyanin. It contains 

 amoeboid cells, and in Solen legumen and 

 Area none discoidal corpuscles charged with 

 haemoglobin, which gives the blood a red 

 colour. 



The relation of the ventricle to the rectum 

 varies in different forms. As a rule the 

 rectum perforates the ventricle, but it passes 

 dorsal to it in Area, Nucula, and Anomia, 

 ventral to it in Teredo and most species of 

 Ostrea, and has a tendency to the latter 

 arrangement in Pinna, Perna, and Avicula. 



The vascular system does not communicate 

 either with the exterior or with the peri- 

 cardium. 



In Area (Fig. 261) there are two ven- 

 tricles, and each gives off a single artery, 

 which divides at once into an anterior and 

 posterior vessel ; the two anterior arteries 

 fuse to form a single anterior aorta, and 

 the two posterior unite into a posterior 

 aorta. 



In Ostreidae (Fig. 262) the auricles have partly coalesced, though their outer 

 portions have remained distinct. 



Organs of respiration. The Lamellibranchs possess two ctenidia 

 attached to the ventral surface of the body, one on either side of the 

 foot. In the simplest cases (Protobranchiatd) the ctenidium has the 

 typical Molluscan form (Figs. 263, 264 A) ; that is to say, it consists 

 of a vascular axis bearing two rows of hollow lamellate processes or 



Ad 



FIG. 261. Dorsal view of Area noae 

 removed from the shell (after 

 Grobben). The double pericardial 

 cavities P are opened, and the 

 rectum D turned forward. VS 

 anterior, HS posterior adductor 

 mnscle ; VR anterior, HR posterior 

 retractor ; V ventricle ; A auricle ; 

 Ao anterior, Ao' posterior aorta; 

 N kidney. 



