22 MOLLUSCA. 



ventricle of the heart, passes over (dorsal to) the posterior adductor 

 muscle to open at the hind end of the body into the mantle cavity 

 at the end of a projecting papilla. 



The circulation is effected by an arterial heart, which is enclosed 

 in a pericardium and lies in the dorsal middle line slightly in front 

 of the posterior adductor muscle. The heart consists of a median 

 ventricle, which is perforated by the alimentary canal, and of two 

 lateral auricles, through which the blood enters the ventricle. The 

 ventricle of the heart of Area is peculiar in being double; the efferent 

 aortas, however, unite to form an unpaired vessel. The ramifications 

 of the anterior and posterior aorta lead the blood into a compli- 

 cated system of lacunae in the mantle and in the interspaces 

 between the viscera. These, which coincide with the body cavity, 

 represent the capillaries and finer venous vessels ; while, by some 

 observers, they have been regarded as a true capillary and venous 

 system. The chief venous sinuses are two lateral sinuses placed at 

 the base of the gills, and a median sinus into which the lacunae of 

 the foot lead. From these part of the blood passes direct into the 

 gills; the main part, however, first passes through a network of 

 canals in the walls of the kidney or organ of Bo j anus, as through a 

 kind of portal circulation, and thence into the gills, whence it is 

 returned as arterial blood to the auricles of the heart. Water is 

 said to enter the circulation through openings in the foot and to 

 become mixed with the blood. Nevertheless the erectile networks of 

 the foot are blood-lacunae. 



Organs of respiration. There are usually two pairs of branchial 

 leaflets (gills), which begin behind the labial palps and pass back- 

 wards along the sides of the body. The outer surfaces and the 

 interlamellar water-spaces of these branchial leaflets are covered 

 with cilia, which keep up a continuous flow of water over the gills. 

 The outer gill, viz., that lying next the mantle, is usually con- 

 siderably the smaller of the two. It is often completely absent, so 

 that the number of the gills is reduced to a single pair. Sometimes 

 the gills of the two sides fuse with one another across the middle line 

 in the posterior region, and may in extreme cases represent a sack, 

 like the branchial sack of the Ascidians (Clavagella). 



The most important of the excretory organs the organ of 

 Bojanus, so-called after its discoverer is a paired, glandular sac 

 with folded walls, and of an elongated oval form, whose cavity 

 communicates with the pericardium (fig. 498). The substance of 

 this gland, which functions as kidney, is composed of a yellow or 



