198 THE ACEPHALA. <§> 183, 



A. The central nervous mass is composed of three pairs of principal 

 ganglia, as follows: Pa?- ant.eriiis or lahiale, Par posterius^ and Par 

 inferms or abdomhmle. This last pair is extraordinarily developed with 

 those species which have a foot, and has, therefore, received also the name 

 of Par pedale. 



The ganglia of the Par anterms are situated one on each side of the di- 

 gestive canal, and are connected by a filament which extends arcuately 

 over the circumference of the oral cavity. <■'' They send off two long; 

 nerves, which pass along the back to the Par posterius, which is the larg- 

 est of all and usually situated upon the anterior surface of the posterior 

 adductor muscle. Its two ganglia are either blended together, or connected 

 by a transverse commissure. <'^'> In this way, the anterior and posterior 

 pairs with their commissures form a kind of oesophageal ring which sur- 

 rounds the base of the abdomen. 



The inferior or pedal pair is situated at the point where the base of the 

 foot is joined to the abdomen. Its ganglia are contiguous upon the median 

 line, or are blended together into one.^''* They connect also with the labial 

 ganglia by two nerves, thus forming a second oesophageal ring.'**' 



Besides these principal ganglia, there are others, smaller, and situated in 

 various parts of the body. But these are not constant, for their presence is 

 always due to an unusual development of the muscular system. 



B. The peripheric nerves arise almost exclusively from the three pair of 

 principal ganglia, for the nerves of connection do not usually give off 

 branches. The few and very delicate filaments which sometimes pass off 

 from these last, belong, probably, to the splanchnic system, for the princi- 

 pal ganglia appear to furnish only sensitive and motory nerves ; these are 

 usually distributed in the following manner : 



The Par anterms sends nerves to the anterior part of the mantle,*^' to 

 the anterior adductor muscle, and to the tentacles of the mouth and its cir- 

 cumference. 



The Par posterius sends two very large trunks to the branchiae ; also 

 Other nerves to the lateral and posterior part of the mantle, *^°> to the pos- 

 terior adductor muscle, and delicate filaments to the heart and rectum. 



5 The length of the arc of this filament of con- leT''s Arch. 1840, p. 33, Taf. IFi. fig. 3, g. ; and 



Bection depends upon the position of the two labial Blanchard, loc. cit. p. 336, PI. XII. fig. 3, a. b.), 



ganglia. Thus with Pecten, where these ganglia between these ganglia with which it is in connec- 



are situated unusually in the rear, it is very long tion by commissures. 



and very arched ; while with Pholas, and Solen, 8 Of these three ganglia, the Par anterius has 



where they are close upon the oral opening, it is often been regarded as the brain. But others have 



short. With Fenus, and Mactra, these ganglia lie rather taken the Par posterius for the principal 



80 close to each other, that this anastomotic fila- nervous mass. For my own part I think that all 



ment is replaced by a very short, transverse com- three, together with their commissures, correspond 



missure. to the pharyngeal system of Gasteropoda. 



C The Par posterius, which, from its relations M The anterior nerves of the mantle of Solen, 



to the branchiae, is also called the Par branchiate, which is prolonged far beyond the oral opening, 



is blended into a single ganglion in those S])ecie3 and is strengthened by a muscular mass — have 



whose branchiae are united at their lower part ; as ten to twelve ganglia lying along the border of the 



Unto, Anodonta, Mactra, Mya, Solen, and Pho- mantle. With Pecten, the mantle-nerve has also 



las. On tlie other hand these ganglia are separate, a small ganglion upon the muscular mass which is 



and connected simply by a transverse commissure, found upon each side of the anterior border of this 



with those whose branchiae are isolated ; as Os- organ ; see Blanchard, loc. cit. p. 333, PI. XII. 



trea, Pecten, Avicula, Mytilus, Lithodomus, fig. 1, f. {Solen), fig. 3, c. {Pecten). 



Modiola, and Area. 1'^ In the mantle of Ostrea, Spondi/lus, Pecten, 



7 According to the earlier Zootomists, the Par Lima, and in general those species in which its 

 pedale was wanting in those species which have borders have numerous sensitive organs, the 

 no foot, although careful investigation has shown branches of the anterior and posterior mantle- 

 that there is a pair corresponding to the Par infe- nerves unite and form a common marginal nerve 

 rius. I refer to that found with Ostrea {Brandt, whose size depends upon the number of the sensi- 

 loc. cit. Tab. X.X.KVI. flg. 11, a. o.), directly behind live organs to which it sends filaments, 

 the labial ganglia, and with Petiten {Grube, Mut- 



