GASTEROPODA. 303 



median and superior pair passes along the sides of the oesophagus, con- 

 verges and meets below to form a pair of ganglions in close contact 

 with one another which supply the foot and viscera : these are evi- 

 dently analogous to the bilobed pedial ganglion of the Mytilus. The 

 lateral and inferior filaments pass downwards to join two widely sepa- 

 rated branchial ganglions, analogous to those situated on the posterior 

 adductor in the Mytilus. We observe, however, a considerable dif- 

 ference in the relative positions of the pedial and branchial ganglions 

 in the limpet ; the latter have advanced into close contiguity with the 

 pedial ganglions, and are connected with them by the same transverse 

 chords, which in the Pecten and Mytilus serve merely to bring the 

 branchial ganglions themselves into mutual communication. 



We thus observe in the lowest and least locomotive Gasteropods a 

 tendency in the nervous system to be aggregated at the fore-part of 

 the body, the cerebral ganglions rising more to the upper surface of 

 the now well-developed head, and the branchial and pedial ganglions 

 beginning to concentrate themselves about the mouth. But this 

 march of development does not prevent the homologies of the different 

 ganglia from being satisfactorily traced. In the limpet there is a dis- 

 tinct head and mouth, with organs of special sense ; and besides the 

 large antennal and small ophthalmic branches given off from the 

 cephalic ganglia, we find also superadded ganglia, having evident 

 relation to the muscular mouth and pharynx and to tlie complex 

 tongue, which are so many accessory parts appended to the simple 

 opening of the gullet, with which the alimentary canal commences in 

 the bivalves. The additional ganglia in question are placed below 

 the pharynx, and are brought into communication with the sentient 

 centres by a filament continued downwards and forwards from each 

 of these ganglia : they also inter-communicate by a loop which 

 forms a third azygos rudimentary ganglion beneath the oesophao-us 

 completing an anterior ring corresponding to that which is formed by 

 the means of the pedial ganglion posteriorly. 



The ganglions corresponding to the pedial pair in the Bullcca appear 

 not to be joined together by a transverse band, but to be connected 

 only with the branchial ganglion, and through them with the 

 cerebral ones. The three are placed so close together, that Cuvier 

 describes them as forming one mass. There are two pharyngeal 

 ganglia formed upon filaments descending from the cerebral ganglions. 

 The labial ganglions, which are devoloped in addition to tlio pedial 

 ganglions, originate from the latter in the Bulla lignaria, and are 

 connected with the cerebral ring only through them. 



In the Haliotis, the superior or oesophageal part of the oesophageal 

 circle is still a simple commissural chord. The sides of the circle are 



