THE LAMELLIBRANCHIA 213 



The shell of Lamellibranchs is formed of two valves, each 

 corresponding to a lobe of the mantle. The internal layer of the 

 shell is secreted by the whole external surface of the mantle, but 

 the external layer is secreted only by the thickened mantle edges. 

 The internal layer is often nacreous, and may exhibit pathological 

 products called "pearls," which are formed by the secretion of 

 nacreous substance by the mantle round foreign bodies. These 

 are generally of parasitic origin : the scolex of a Cestode forms the 

 nucleus of a pearl in the genus Meleagrina of Ceylon and the Gambier 

 islands; the larvae of Trematodes form similar nuclei in various 

 European Lamellibranchs. 



Though they are primitively symmetrical and commonly remain 

 so, the valves become very asymmetrical in some species of Area, 

 in the Anomiidae, Pecten, Ostraea, C&rbula, Chama, Pandora, Myocliama, 

 the Rudistae (Fig. 244), etc. In certain somewhat modified forms 

 in which the foot, though more or less large, is feebly retractile, 

 the valves do not meet and fit perfectly together along the ventral 

 edge and are "gaping," as may be seen in the Pholadidae, Gastro- 

 chaenidae, etc. But with the exception of Cklami/doconcha and 

 Sdoberetia, in which the shell is internal, the valves fit together 

 perfectly along the dorsal border, and are articulated with one 

 another by a system of teeth and sockets which collectively form 

 the hinge (Fig. 189), and only tend to be atrophied in forms whose 

 valves have little mobility, especially in boring species. The valves 

 are additionally united (except in the Pholadidae and Teredinidae, 

 hence named Adesmacea, and a few other forms) by a ligament of 

 a chitinous nature. This ligament is primitively continuous with 

 the shell, and is, in fact, the uncalcified portion of the pallial cuticle, 

 that is to say, of the originally single shell. The ligament finally 

 becomes external (Fig. 189) or internal; in the latter case it is a 

 " resilium." Its action is antagonistic to the adductor muscles, and 

 consequently it causes the valves of the shell to gape. 



In the youngest stages of the Protobranchia, Filibranchia, and 

 various Eulamellibranchia, a series of little transverse denticulations, 

 constituting a primitive hinge or provinculum (Bernard), is developed 

 on each side of the ligament, or at any rate behind it in forms 

 devoid of an anterior adductor muscle. The permanent hinge teeth 

 are only formed at a later period, by the growth of distinct laminae 

 on the surface of the hinge. Thus, in the typical Eulamellibranchia, 

 the first lamellae originate at the extremities of the hinge surface, 

 below the provinculum, and grow towards the centre of the hinge 

 area ; the internal ends of the anterior lamellae become hook- 

 shaped, and their hooks become separated from their external ends ; 

 the latter form the anterior lateral teeth, while the hooks become 

 the cardinal teeth, and the posterior lamellae give rise to the 

 posterior lateral teeth. 



