20 MOLLUSCA. 



while the two outer layers are formed only by the free edge of tho 

 mantle. The growth of the shell is effected in two ways; (1) by 

 additions to the internal laminated layer, whereby the shell increases 

 in thickness; (2) by additions to the prismatic and horny layers, 

 whereby it increases in superficial extent. Accordingly the outer 

 coloured part of the shell, which is composed of vertical prisms and a 

 horny cuticle, when once formed cannot increase in thickness ; while 

 new concentric layers are constantly being added to the internal 

 colourless mother-of-pearl layer during the whole life of the animal. 

 The mantle-secretion gives rise in the so-called pearl-mussel (Mdea- 

 yrina, Unio margaritifer), to the formation of pearls. 



The foot is completely absent in comparatively few of the Lamelli- 

 branchiata, and only in those which have lost the power of locomotion 

 (Ostrea, Anomia). In many forms, principally in the larva (Unio), less 

 frequently in the adult (Mytilus), the foot possesses a byssus gland, 

 which secretes silk-like fibres, by which a temporary or permanent 

 attachment of the animal is effected. The form and size of the foot 

 vary very considerably, according to the special kind of locomotion. 

 The foot is most frequently used for creeping in sand, and then is 

 hatchet-shaped; in other cases it is spread out laterally and its 

 creeping surface has the form of a disc. More rarely it is of a large 

 size and bent, in which case it serves for springing movements in the 

 water (Cardium). Some Lamellibranchs possess a linear club-shaped 

 or cylindrical foot (Solen, Solenomya), and move by rapidly retracting 

 the foot and ejecting water through the siphons. Many use the foot 

 for burying themselves in mud ; others bore into wood (Teredo) or 

 hard rock (Pholas, Lithodomuus, Saxicava, etc.), for which purpose 

 they push themselves against the rock with their short blunt foot 

 (Pholas, Teredo), and use the hard and often finely serrated edge of 

 their shell as a grater, giving it a rotatory movement. According to 

 Hancock, the foot and edge of the mantle at the anterior end of the 

 gaping shell are beset with siliceous crystals, and effect the excava- 

 tion of the rock after the manner of a file. 



The nervous system presents three pairs of ganglia, the cerebral, 

 pedal, and visceral ganglia. The visceral ganglia are connected 

 with the cerebral by a longer or shorter commissure on each side 

 (figs. 496 and 498). Since there is never a distinct head, and sense 

 organs do not appear on the anterior region of the body, the brain 

 (cerebral ganglia) is proportionately little developed. Its nerves 

 supply mainly the region round the mouth and the mantle, to which 

 two large nerves are often distributed. The two halves of the brain 



