32 MOLLUSC A. 



gills, and from the surface of the large muscle, where 

 there occurs a pair of nervous swellings or centres of 

 distribution of the nervous stems which are united by 

 a cross-bar or nerve. From this pair of ganglia the 

 various nerves radiate out to the great muscle^ the 

 mantle, the viscera, and other parts, while two large 

 branches run forward upon either side to the bases of 

 the gills, and bend quickly back again, following the 

 ridge of the base to the posterior end, and giving off 

 a great number of lateral branches.* 



A pair of branches concealed in the interior of the 

 body connect this pair of ganglia with another situated 

 under the palpi and near the mouth. These ganglia 

 give off branches to the mouth parts, the foot, mantle, 

 and gills. 



Though supplied with nerves to such an extent that 

 each of the papillae of the fringes on the two edges of 

 the mantle has its own nerves, there are no special 

 organs of taste or smell or sight. Their places are 

 supplied by these papillae, which are supposed to act 

 as organs of touch, as well also as the palpi. There 

 are no eyes. 



The related forms of the oysters are, for the most 

 part, unsymmetrical animals, and have no feet or digging 

 organ for constructing burrows, or siphons for reaching 

 up to the water above if accidentally buried too deep 

 in the mud. The oyster, therefore, unlike the clam, 

 cannot live anywhere below the surface mud, and must 

 He upon one side. It is, however, when first born, a 

 free, moving animal ; and at that time, and for a long 



* These are shown as black lines in Fig. 5, n. 



