134 FRESH- WATER MUSSEE —COMMON MARINE MUSSEL 



4. Remove the mantle and gills from the left side. Note and draw the 

 following parts : — 



(a) The foot, a large muscular organ situated at the anterior end of the 



animal. The foot is axe-shaped (Pelecypoda). [In Mytilus there is a 

 byssus inserted on the posterior surface of the base of the foot.] 



(b) The pair of labial palps on each side of the mouth. 



(c) The anterior adductor, anterior retractor, protractor, posterior re- 



tractor, and posterior adductor muscles. [In Mytilus the anterior 

 adductor and protractor muscles are absent.] 



(d) The gills of the right side. 



(e) The mantle lobe of the right side, lining the shell. 



Pericardium 



Digestive gland 

 Aiitfiioi letiactoi 



Fig. 45. 



Labial 



-The Fresh-water Muasel (Anodonla). (Shell removed. Part of the mantle of the left side 

 has been taken away so as to expose the gills, foot, etc. 



(/) The visceral mass forming the upper two-thirds of the large foot, a 

 laterally compressed, oblong mass. The lower third is the muscular 

 portion of the foot, or foot proper. 



(g) The pericardium, lying along the dorsal surface, beneath the huige- 

 ligament. Within it is the heart, which consists of two auricles and 

 a ventricle. The ventricle surrounds the rectum, which passes through 

 the pericardium. 



(h) The paired kidney or " organ of Bojanus," lying just beneath the peri- 

 cardium, with which it communicates through two crescentic reno- 

 pericardial openings (ncphrostomes) at the anterior end. [In Mytilus 

 the kidneys lie on the floor of the pericardium on either side of the 

 heart.] 



(i) Keber's organ or the pericardial gland, a glandular, reddish-brown 

 mass, lying in the anterior region of the body just in front of the 

 pericardium, [hi Mytilus this gland does not occur.] 



