310 Mollusca. 



ofclier in the posterior, regiorij and are attached to the interior 

 of the shell ; occasionally one such muscle only is present [e.g., in the 

 Oyster). The attachment of an adductor to the shell is indicated by a 

 sharply defined area, a muscular impression, of which there 

 are two on the inner side of each valve. Besides these, there are 

 frequently smaller impressions, which correspond with the attach- 

 ments of the pedal muscles; further, the so-called pallial line, 

 from which the muscle-fibres of the pallial muscle arise ; in those 

 Lamellibranchs which have no syphons, it runs parallel to the edge of 

 the shell some distance from it (integripalliate) ; in those which have 

 syphons it usually traces out a notch, extending from behind forwards 

 (sinupalliate) (Fig. 256, B) ; and along this the muscles of the 

 syphons arise, being specially developed portions of the pallial muscle; 

 since these muscles arise more anteriorly, the syphons can be with- 

 drawn into the shell. Three layers may be usually distinguished .in 

 the shell: an outer horny layer in some {e.g., Mussels), very 

 definite^ in others, indistinct; and two layers consisting principally 

 of calcium carbonate, which make up the chief part of the shell ; 

 of these, the inner layer is sometimes iridescent (mother of 

 pearl) . 



The flexible band which holds the two valves of the shell together, 

 the so-called ligament, consists of an outer, flexible, but inelastic 

 layer (an extension of the outer layer of the shell), and an inner, 

 elastic portion of radially arranged flbres. In many forms, this 

 ligament has a large, very convex surface, exteriorly ; and is termed 

 an external ligament: in others, it is enclosed between the 

 upper edges, displaying a very small portion externally, whilst it is 

 convex inwardly, and is then termed an internal ligament. 

 Its action is essentially the same in the two cases (c/.. Pig. 257). If 

 a shell, furnished with an internal ligament, is closed by the 

 contraction of the adductor muscles, the inner elastic portion of the 

 ligament is compressed, and when the muscles relax, the valves of the 

 shell are once more driven apart. In the shells provided with an 

 external ligament, the elastic substance of the ligament undergoes 

 a compression, or arching together, within the outer inelastic sheath, 

 and on the relaxation of the muscles, the effect is the same as in shells 

 with internal ligaments.* The action of the ligament is purely 

 mechanical, and occurs after the death of the animal just as in life. 

 It must also be mentioned that its position is either directly below the 

 umbo, or more usually, posterior to it. Only in very incompletely 

 developed shells {e.g., Teredo) is the ligament altogether wanting, and 

 the shell-valves completely separate. 



* The usual statement, that a stretching of the external ligament must occur on 

 closure of the shell, is incorrect. 



