XII 



PHYLUM MOLLUSCA 



679 



thick brown periostracum and large external ligament, the elon- 

 gated branchial or inhalant aperture, the long, compressed foot, 

 and the absence of a byssus, place them among the Unionidae. 

 Anodonta is distinguished from Unio by the absence of hinge- 

 teeth. 



3. GENERAL ORGANISATION. 



The most important variations in structure in the present class 

 are connected with modifications of the gills, the foot, the muscular 

 system, and the siphons. With the structure of the muscles and 

 of the siphons are correlated important variations in the shell 

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

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



XIV 



-VIII 



, 



FIG. 585. Anatomy of Pecton. I, palpi ; II, foot ; III, aperture of gonad into kidney ; IV 

 external renal aperture ; V, male, and VI, female portion of gonad ; VII, pallial 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.) 



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

 Area, 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 (Mytilus 

 edulis), the anterior adductor becomes greatly reduced and the 

 posterior correspondingly enlarged ; and in another species of the 

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

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



