192 



THE SEA SHORE 



taking first the bivalves, the shells of which are found in great 

 variety along our shores. 



The general nature of a lainellibranch is easily made out by 

 the examination of one of the common species that may be 

 obtained alive on any part of the coast, such as the Edible Mussel, 

 the Cockle, or the Oyster, and the reader will do well to secure 

 a few specimens and examine them with the aid of the following 

 description of the principal distinguishing features. 



The shell is formed of two valves, united by a hinge which is 

 sometimes of the simplest possible description, but which often 

 exhibits a beautiful arrangement of interlocking teeth. A ligament 

 of flexible and elastic substance often holds the two valves together. 



FIG. 127. SHELL OF THE PRICKLY COCKLE (Cardium aculeatum) 

 SHOWING UMBO AND HINGE ; ALSO THE INTERIOR SHOWING THE TEETH 



The reader has probably observed that the valves of a dead 

 lamellibranch usually gape. This is due either to the pull exerted 

 by a ligament that is attached to the valves outside the hinge, or 

 to the pressure of an internal cartilage which unites the valves 

 within, and which is compressed when the shell is closed. When 

 the animal is alive, it has the power of closing its shell by the 

 contraction of the adductor muscles, to be presently described, and 

 when the valves are brought together by this means the external 

 ligament is more or less stretched, or the cartilage within, which is 

 also an elastic material, is compressed. 



