PTEROPODA AND GASTROPODA. 547 



A few Gastropods, for example, are symmetrical, more or less flat, 

 or depressed; others are compressed; the majority are contorted and 

 lose their symmetrical form in an oblique twist : there are other di- 

 versities of organic structure which more immediately affect the con- 

 dition of the nervous system, for some species possess both eyes and 

 tentacles, whilst others are blind and akerous. 



The nervous system has a distinct fibrous neurilemma, often 

 charged with pigmental cells. The ganglion-cells are often pedun- 

 culated, and have usually a very large nucleus composed of obscure 

 granules, in the midst of which are usually two to four transparent 

 nuclei. The central part of the system surrounds the oesophagus, 

 and consists of different parts, of which that which is above the tube, 

 and which usually includes two contiguous ganglia, is called the 

 brain. 



In the limpet (Patella), and bubble-shell (Bulla), we find that the 

 cerebral ganglions, as in the bivalves, are still distant from each 

 other, and situated at the sides of the oesophagus, connected together 

 by a nervous chord or commissure which arches over that tube : 

 from these ganglions two filaments proceed backward on either side ; 

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

 converges and meets below to form a pair of ganglions in close 

 contact with one another which supply the foot and viscera : these 

 are evidently homologous with the bilobed pedial ganglion of the 

 Mytilus. The lateral and inferior filaments pass downwards to join two 

 widely separated branchial ganglions, homologous with those situated 

 on the posterior adductor in the Mytilus. We observe, however, a 

 considerable difference 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 Pecten and Mytilus 

 serve merely to bring the branchial ganglions themselves into mutual 

 communication. 



We thus observe in the lowest and least locomotive Gastropods 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 (infero-pharyngeal) ganglia, 

 having evident relation to the muscular mouth and pharynx and to 



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