208 NERVOUS SYSTEM. 



and represented by Poll, as extending over the branchiae and 

 the posterior portion of the mantle. These posterior ganglia, 

 situate on the great adductor muscle, and sending forwards 

 branches to the gills, are very remote from each other 

 in the avicula; they are separate in the mactra as in the 

 mytiluSy they are partially joined into a quadrilobate mass 

 in the cardium and the solen, they form a single gan- 

 glion in the spondylus as in the pecten, and they form 

 by their union a transverse thick nervous band on the 

 large abductor of the pinna. The first sub-cesophageal pair 

 of ganglia (88 a. b.) with their numerous anterior branches 

 and their two columns extending backwards to the pos- 

 terior ganglia (88. e) have been accurately traced and re- 

 presented by Poli in different species of solen. Small 

 ganglia are observed in the conchifera, as in the tunicata 

 and in the articulated classes, on other nerves besides the 

 two great sensitive columns. The visceral or sympathetic 

 nerves appear to receive their principal branches from the 

 nerves of the first sub-cesophageal pair of ganglia, as they 

 have been long known to receive their principal trunks 

 from those of the two corresponding lateral ganglia of the 

 stomach in the Crustacea. 



From the great development of the organs of the senses and 

 of mastication at the entrance to the oesophagus in the gas- 

 teropods, their nervous axis is much more concentrated and 

 developed in that situation than it is in the conchifera ; 

 and from this general advancement of the great nervous cen- 

 tres to the head of these animals, we commonly observe a 

 proportional diminution in the extent of their two symme- 

 trical sub-ventral nervous columns. In the short and broad 

 bodies of the gasteropods, the symmetrical columns are still 

 generally separated from each other by a variable space along 

 the median plain, as in the conchiferous mollusca, but as the 

 general form and structure of the animals of this class vary 

 remarkably, we find a corresponding diversity in the form 

 and disposition of the great centres of nervous energy. In 

 the simple form of the nervous system presented by the 

 carinaria mediterranea (Fig. 89,) there is a close analogy 

 with the ordinary disposition of the symmetrical detached 

 nervous columns along the ventral surface of the abdomen in 

 the inhabitants of bivalve shells. Lobed ganglia (g. h,) are 



