136 FRESH- WATER MUSSEL -COMMON MARINE MUSSEL 



( j) The digestive gland or liver, a large organ surrounding the stomach and 

 the anterior end of the intestine. 



5. Expose the heart by opening the pericardium. Note the heart, consist- 

 ing of a single ventricle and a right and a left auricle. The ventricle is a thick- 

 walled muscular organ surrounding the rectum. Each auricle is a thin-walled 

 triangular sac. the base extending along the dorsal border of the gills, the apex 

 opening into the ventricle. From the anterior end of the ventricle the anterior 

 aorta arises, and runs forwards along the dorsal surface of the rectum to supply 

 the visceral mass. From the posterior end of the ventricle the posterior aorta 



Renal ain'vtiir 

 Genital apei'tin'6 



lleiuj-pfnicai-diul aperture 

 Anterior aorta 



l.eft aurieie 



Veiitricii 



DiKestiv 

 Slomaeli 



rerir'arriintil 



Rectnni 



PoKteri'n- aorta 



Cerebral f;ang 



Cei-ebro-visr:e[al (■oiineft,ive 



Fig. 46. — The Fresh-water Mussel (Aiwdoiild.) dissected from the left side to show the general 

 structure. After Parker and Haswell. 



arises, and passes backwards along tlie ventral surface of the rectum to .supply 

 the hinder part of the body, [hi Mytilus the auricles are not so apparent, and 

 the aortce are inconspicuous.] 



6. Examine the kidney of the left side. It is a wide, thin-walled tube con- 

 sisting of twf) parts, the one part douljled over on to the other so as to lie dorsaUy 

 to it. The \'entral part forms the kidney proper, while the dorsal forms the 

 ureter or urinary bladder. The ureter opens to the exterior by a small aperture 

 situated Ijctween the inner gill and the visceral mass. Just below this aperture 

 may be seen the much smaller genital aperture. The kidney proper opens 

 anteriorly into the pericardium by the crescentic reno-pericardial aperture or 

 nephrostome. \In Mytilus the kidney has a, simpile structure, and from its 

 posierior end the ureter passes backivards to unite with its fellow from the other 

 kidney, the single duct thus formed opening on the dorsal surface of the posterior 

 adductor muscle.] 



