NERVOUS SYSTEM. ^O/ 



size, and correspond in their development with the presence 

 or the magnitude of the foot which receives branches from 

 them. The columns which connect these two pairs of ganglia 

 are separated from each other on the median plain by 

 a space, which varies according to the lateral extension 

 of the trunk in the different species ; they are generally 

 parallel and near to each other in their course. The two 

 symmetrical columns of ganglionic and simple nerves, 

 continuing backwards beneath the ovary to the inferior 

 surface of the great adductor muscle of the valves, 

 meet with a third pair of infra-abdominal ganglia 

 (88. e.) which are the most approximated to each other, 

 and are often united into a single lobed ganglionic mass, 

 as in the pecten maximus. In this large pecten it is 

 easy to perceive that the motor portion of each of the 

 two converging columns passes laterally over the surface 

 of this large compound ganglion placed on the median 

 plain, under the middle of the large adductor muscle 

 of the valves. This pair of ganglia (e) appears to vary 

 in size with the magnitude of the adductor muscle, and 

 the extent of the palleal margins and branchke which receive 

 nervous branches from it. The columns which pass backwards 

 from the posterior ganglia, soon divide into numerous 

 branches which supply this part of the trunk, and the 

 largest nerves (/.), continued upwards along the adductor 

 muscle from the columns, are observed to extend, on each 

 side of the rectum, to the margins of the mantle, and to 

 supply the ciliated and fringed orifices of the abdominal 

 and thoracic cavities, and they send large branches to the 

 branchiae. The ganglia, which are most obvious on first 

 opening the valves of the conchifera, are the posterior pair 

 placed on the adductor muscle, and these have generally 

 bi'CMi described and figured by Poli as the centre of this 

 system, which he mistook for the chyliferous system of 

 these animals; the same pair has been uniformly designated 

 and represented by Chiaje as the brain of these acephalous 

 niollusca ; but both these authors have accurately represented 

 their forms and the distribution of their numerous diverging 

 branches. These two ganglia are connected by a trans- 

 verse band on the posterior adductor muscle of the broad 

 area noa, the nerves of which have been minutely traced 



