XII 



PHYLUM MOLLUSC A 



697 



which arc of great systematic value, especially in cases where, as 

 with fossils, the shell is the only part available for examination. 



In all the Protobranchia, some of the Filibranchia, such as 

 Arm, and all the Eulamellibranchia and Septibranchia, there 

 are two almost equal-sized adductor muscles, as in Anodonta. 

 In many Filibranchs, such as the common sea-mussel (Mytilits 

 ci/iilis), the anterior adductor becomes greatly reduced and the 

 posterior correspondingly enlarged ; and in another species of the 

 same genus (M. lotus) the anterior adductor has completely 

 atrophied, the function of closing the shell being performed by the 



XIV 



Fig.. 575. — Anatomy of Fecten. I, palpi ; II, foot; III, aperture of gonad into kidney; IV, 

 external renal aperture; V, male, and VI, female portion of gonad; VII, pallia! eye; 

 VIII, visceral ganglion ; VIII', gill ; IX, anus ; X, striated portion of adductor ; XI, smooth 

 portion; XII, retractor of foot; XIII, heart; XIV, liver; XV, stomach. (From Pelseneer's 

 Mollusques.) 



great posterior adductor alone. In* Anomia and in the Pseudo- 

 lamellibranchs there is a single immense adductor (Fig. 575, X, XI) 

 placed nearly in the middle of the greatly shortened body, and 

 known to represent the posterior adductor — both from the fact that 

 the rectum passes over it, and from the circumstance that, in the 

 embryo Oyster, two adductors are present, the anterior of which 

 atrophies, while the posterior enlarges to form the single muscle 

 of the adult. 



These peculiarities in the muscular system bear their mark 

 upon the shell, in which impressions corresponding to the 



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