648 



ZOOLOGY 



SECT, 



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

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



In all the Protobranchia, some of the Filibraiichia, 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 

 cdulis), the anterior adductor becomes greatly reduced and the 

 posterior correspondingly enlarged, and in another species ol the 

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

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



XIV 



XV 



IX 



IV 



FIG. 537. Anatomy of Pecten. I, palpi; II, foot ; III, aperture of gonad into kidney ; IA ... 

 external renal aperture; A T , male portion of gouad ; A'l, female portion; A'H, pallial eye: 

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

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

 Hollusques.) 



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

 lamellibranchs there is a single immense adductor (Fig. 537 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, tAVO adductors are present, the anterior of Avhich 

 atrophies, Avhile the posterior enlarges to form the single muscle 

 of the adult. 



These peculiarities in the muscular system bear their mark 

 the shell, in Avhich impressions corresponding to the 



