CHAPTEE 



THE FRESH- WA TER MUSSEL. 



THE Common Fresh-water Mussel (Anodonta cygnea) is enclosed 

 in a bivalve shell which constitutes its exoskeleton. This shell 

 (Fig. 89) is greenish brown in colour and rough ; of an elongated 

 oval form blunter at one end, more tapering at the other ; and 



ex. si jLjf iu. si. 



ill. 



Fin. SO. MUSSEL : SHELL AND EXTERNAL CHARACTERS. 



". mi. i. Anterior adductor impression, a. r. i. Anterior retractor impres- 

 sion, ex. si. Exhalent siphon. /. Foot. f. e. Inturned edge. in. si. Inhalent 

 siphon. I. r. i. Lesser retractor impressions. Ig. Ligament, p. ad. i. Pos- 

 terior adductor impression, p. p. i. Protractor pedis impression, p. r. i. 

 Posterior retractor impression, pal. 1. Pallial line. urn. Urnbo. 



marked with a series of concentric lines, each of which was at 

 one time the margin of the shell. The starting-point or origin 

 of this series of curved lines, the spaces between which represent 

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