ANATOMY OF A FRESH-WATER MUSSEL. 53 



the anterior retractor muscle, and divides into a visceral and a 

 pedal artery, the former of which supplies 

 blood to the digestive and generative organs, II 



the latter to the foot, besides giving off a _^ ' V^_ 



labial branch to the labial palps, and a pallial ~^\ T^~ 



branch to the mantle and anterior adductor ] f 



muscle. The posterior aorta gives branches Fig I9 ._ Diagram of the 

 to the mantle, body-wall, and posterior ad- heart of a Mussel. 

 ductor muscle. These arteries break up 

 into numerous lacunar spaces, from whence the blood finds its 

 way into the vena cava which lies under the pericardium. From 

 this vessel the blood passes to the organs of Bojanus, in which 

 the blood-vessels form an irregular plexus, and thence by the 

 afferent branchial vessels to the ills, where it becomes oxidised, 

 losing carbon dioxide and gaining oxygen. From them, the now 

 arterialised blood is carried by the efferent branchial vessels to 

 the efferent branchial sinus, which expands to form the auricle. 

 The direction which the blood takes may then be arranged in a 

 tabular form, thus : 



1. Ventricle. 



2. Anterior and posterior aortse. 



3. Arterioles and lacunar spaces. 



4. Vena cava. 



5. Organ of Boj anus. 



6. Gills. 



7. Auricles. 



8. Ventricle. 



The blood or haemo-lymph consists of a colourless plasma, in 

 which colourless corpuscles (leucocytes} float. The extension of 

 the foot for the purposes of locomotion is due to a congestion of 

 the lacunar spaces in that organ. The vein which conveys the 

 blood from the foot is surrounded by a muscular sphincter, which, 

 when the animal wishes to walk, contracts, and so prevents any 

 return of blood, while the heart continues at the same time to 

 pump blood into the lacunar spaces. A peculiar plexus of vessels, 

 known as the pericardial gland or Organ of Keber, exists near 

 the anterior end of the pericardial cavity. Its function is un- 

 known. 



