580 THE INVERTEBRATA 



'over the insertion of the ctenidia into the mantle. A single vessel, the 

 anterior aorta (a posterior aorta is also present in Anodonta), leaves 

 the ventricle, dilates into an aortic bulb and then divides into many 

 arteries. Of these, the most important are the pallial arteries going to 

 the mantle and the arteries forming part of the visceral circulation 

 (the gastrointestinal, hepatic and terminal arteries, the last named 

 supplying the most anterior part of the body including the foot). The 

 arteries break up into a network of vessels in all the tissues and these 



Viscera 



p.n 



Plicate 

 canals 



Fig. 396. Diagram of the circulation in Mytilus to show the greater import- 

 ance of the part of the system in the mantle and pHcate canals. Of the blood re- 

 turned from the viscera a much smaller proportion is sent through the ctenidia. 

 Slightly altered from Field. An. auricle; bl. bladder of kidney opening into 

 the pericardium ; aff.c.v. afferent, eff.c.v. efferent ctenidial vein ; l.v. longi- 

 tudinal vein of kidney; p.ar. pallial artery; pc. pericardium; v. ventricle with 

 rectum, represented by a dotted line, passing through it. 



join to form veins and sinuses which are largely situated on the inner 

 side of the mantle and the superficial parts of the body. The skin, 

 being bathed in water and devoid of any cuticular covering which 

 might hinder diffusion, is a general organ of respiration and the mantle 

 is the most important part of it. Most of the blood from the pallial 

 circulation is returned to the network of vessels in the kidney through 

 the ribbon-like organs, known as plicate canals, which extend along 

 the mantle just above the insertion of the ctenidium. 



