144 MANUAL OF ANIMAL BIOLOGY 



back towards the heart by a contraction of the muscular foot. 

 These are the two main arteries of the body and they give off 

 branches which supply all parts of the body. Attached to the 

 ventricle are two thin-walled chambers, the auricles, one on the 

 right side and one on the left side. These project ventrally and 

 connect with the efferent branchial vessels from the gills. 

 The auricles pass the blood which they receive into the ventricle. 

 In the tissues of the body are numerous blood sinuses similar to 

 those previously noted in the Arthropoda. The main vein of the 

 Clam, the vena cava, lies ventral to the pericardial cavity. The 

 blood from the various regions of the body collects in it, and is 

 then forced through the kidneys and from them carried by the 



AFFERENT BRANCHIAL VESSELS to the gills. (W. f. 49.) 



There is one morphological feature of the Molluscan circulatory 

 system which shows a higher type than any hitherto studied, and 

 that is the presence of auricles in the heart. The blood instead of 

 returning to the pericardial cavity and then being taken directly 

 into the ventricles of the heart through the ostia, as in the Crayfish, 

 is received from the branchial vessels by the auricles which are 

 connected with the ventricle. Therefore, although the pericardial 

 cavity of the Clam surrounds the heart, it does not function as 

 a receiving organ for the blood going to the heart. 



Functional. The general course of circulation of the blood is 

 as follows. Beginning with the ventricle of the heart, the blood 

 is forced either anteriorly or posteriorly through the anterior aorta 

 or the posterior aorta. After passing through the tissues, largely 

 by means of the sinuses, it is finally collected into the large vena 

 cava. This vein is connected directly with the kidneys, so that 

 the blood is next conducted through these excretory organs and 

 during this passage the liquid metabolic wastes are removed. 

 After passing through the kidneys, the blood reaches the gills by 

 way of the afferent branchial vessels. In the gills the respiratory 

 interchange of carbon dioxide and oxygen takes place. The blood, 

 now freed both from the liquid nitrogenous wastes and the carbon 

 dioxide and carrying a fresh supply of oxygen, passes through the 

 efferent branchial vessels into the right or left auricle of the heart, 

 and then into the ventricle, from whence it is driven again over the 

 same course. It should be stated that a small portion of the blood 

 passes from the heart, through a branch of the anterior aorta, into 

 the tissues of the mantle which also serves as an organ of respira- 



