44 ORGANS OF^ SUPPORT, 



generally of two moveable pieces or valves placed on the 

 exterior of the body, and connected together by ligaments 

 and muscles. These valves are composed of carbonate 

 of lime mixed with an albuminous coagulable matter, 

 which is secreted chiefly by the glandular pores on the 

 exterior surface of the lining mantle. The valves grow 

 by the successive addition of larger layers to their inner 

 surface, and the limits of these superadded laminae are 

 commonly marked by distinct striae on the outer surface 

 of the shells. As the calcareous laminae are cast upon 

 the surface of the mantle which envelopes all the soft 

 parts of the body, the shells vary in their form ac- 

 cording to the shape of the enclosed animal, and es- 

 pecially of the secreting glandular portion of their lining 

 membrane. The exterior surface of the valves is covered 

 with a thin layer of the same albuminous matter which 

 unites the particles of the shell : this forms a dense and 

 tough varnish or epidermis, which protects the colouring 

 matter immediately beneath it, and the whole texture 

 of the solid layers. Although most of the conchifera 

 are bivalved, some, as the pholades, possess small sup- 

 plementary pieces at the hinge of the valves, and are 

 thence called multivalves. The relations of these cal- 

 careous shells to the parts of the enclosed animal will be 

 perceived by the annexed sketch of the common muscle, 

 mytilus edulis, Fig. 20, where the soft parts are repre- 

 sented as attached to the right valve (0,) and the left 

 valve (by) is opened and folded upwards. The ligament (c,) 

 connects the two valves to- FIG 20. 



gether, and tends by its 

 elasticity to open them to 

 a limited extent. The liga- 

 ment of bivalvia grows like 

 the shell by successive in- 

 ternal layers, and is placed 

 at the hinge, where there 

 are generally processes 

 of the shell termed teeth, 

 which lock into each other when the valves are closed. 

 The position of the stomach (d,) with the short oesophagus, 

 and the two pairs of long labial tentacula (e 9 ) mark the 



