212 MORPHOLOGY OF INVERTEBRATE TYPES 



opens behind the posterior adductor of the valves into the cloacal 

 siphon. The stomach, intestine and ascending branch of the 

 rectum are imbedded in the muscular tissues of the body, the 

 pyloric end of the stomach and the beginning of the intestine in 

 the base of the foot. 



Excretory system, body cavity and pericardial gland. 

 The majority of the molluscs possess both a primary and second- 

 ary body cavity. The former is represented by the system of 

 sinuses and lacunae by means of which a connection is established 

 between the arterial and venous systems. It always lacks an 

 epithelial covering of its own. The secondary body cavity or 

 ccelome is reduced to the space within the pericard and the 

 lumen of the gonad. It is always lined with its own en- 

 dothelium. The cavity of the pericard communicates directly 

 with the mantle cavity through the excretory system. This 

 system consists of one pair of nephridia (called also organs of 

 Bojanus). They are situated on the dorsal side of the animal 

 between the pericard and the posterior adductor of the valves. 

 Each nephridium communicates by means of an open funnel 

 with the pericardial space, and by means of a nephridiopore 

 or external excretory opening, situated on a minute papilla, with 

 the mantle cavity. The nephridium consists of a wide tube 

 or sac bent on itself. The first part, communicating with the 

 pericardial cavity, is glandular (kidney), the second part is 

 non-glandular and plays the role of a urinary bladder. The 

 right and left nephridiopores are situated underneath the re- 

 spective gills immediately behind the right and left reproductive 

 openings. 



A pericardial gland (often called Keber's organ), probably with 

 excretory function, lies in front of the pericard. In life it has a 

 dark or reddish color. Its secretion is discharged into the peri- 

 cardial space. 



Circulatory system. The circulatory system of Venus, 

 as that of all other lamellibranchs, is an open system in which 

 the veins communicate with the arteries by means of sinuses 



