THE FRESH- WATER MUSSEL (Anodonta cygnea) 

 THE COMMON MARINE MUSSEL {MytUus edulis) 



External Features. 



Make a drawing of the Mussel, viewed from the left side, to show the 

 following parts : — 



(a) The umbo, representing the oldest part of the shell, close to the hinge- 



line, and about a quarter of the length of the shell from the anterior 

 end. The anterior end is more rounded than the posterior end. [In 

 MytUus the umbo is situated at the anterior end, the pointed end, of the 

 shell.'] 



(b) The lines of growth, a number of concentric lines parallel to the free 



margin of the shell and having a common centre at the lunbo. 



(c) The hinge-ligament, occupying the greater part of the straight dorsal 



surface of the shell, and connecting the two valves of the shell together. 

 {d) The brownish-green periostracum, the thin outer horny layer of the shell. 



Fifi. 40.— The Fresh-water Mussel (Anodonta cygnea). x \\. 



(e) The "foot " (if protruded), semi-transparent, tongue-like, and project- 

 ing between the shells, near the antero-ventral end. [In Mytilus the 

 foot is black and has a byssus attached to its posterior surface. The 

 byssus is a long growth of hairs, each tipped with a small flat disc, by 

 which the animal attaches itself.] 



if) The ventral inhalenfc siphon {if protruded), formed by the projection of 

 the mantle between the shells at the posterior end. The lips bear a 

 fringe of small tentacles. 



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