338 MOLLUSCS. 



and backward to all parts of the body. It reaches the tissues 

 by capillaries, and apparently also by lacunar spaces. The 

 venous blood of the head region is collected in an annular 

 sinus round the bases of the arms, and passes towards the heart 

 by a large vein, which divides into two venae cavas covered 

 with spongy appendages. Joined by similar vessels from 

 the apical region of the viscera, each vena cava enters a 

 " branchial heart " at the base of each gill. The'branchial 

 heart is contractile, and drives the venous blood through 

 the gills, whence purified it returns by two contractile 

 auricles into the ventricle. There are valves preventing 

 back flow from the ventricle to the auricles, or from the 

 arteries to the ventricle. Each branchial heart bears an 

 enigmatical glandular structure known as a " pericardial 

 gland," possibly an excretory or incipiently excretory organ. 

 The course of the blood differs from that in the mussel and 

 snail in this, that none returns to the heart except from the 

 respiratory organs. In the spongy outgrowths of the venae 

 cavse, the interesting parasite Dicyema is found. 



Respiratory System. — The blood is purified by being 

 exposed on the two feather-like gills which are attached 

 within the water-washed mantle cavity. These are anything 

 but simple, for their surface is plaited and folded to a much 

 greater degree than we might at first imagine, the water 

 penetrates them very thoroughly, the course of the blood is 

 intricate. At their base there is some glandular tissue, 

 which those impatient with enigmas have credited with 

 blood-making powers. 



The Excretory System is difficult to dissect and to explain. 

 On each side of the anus there is a little papilla through 

 which uric acid and other waste-products ooze out into the 

 mantle-cavity, or we may say into the water. A bristle inserted 

 into either of these two papillae leads into a large sac — ^the 

 excretory or nephridial sac. But the two sacs are united 

 by two bridges, and they give off an unpaired dorsal elonga- 

 tion, which extends as far back as the reproductive organs. 



The dorsal wall of each nephridial sac becomes inti- 

 mately associated with the spongy appendages of the venae 

 cavae, and follows their outlines faithfully. It is likely 

 that waste material passes from the blood through the 

 spongy appendages into the nephridial sacs. 



