204 NERVOUS SYSTEM. 



FOURTH SECTION. 



Nervous System of the Cyclo-gangliated or Molluscous 

 Classes. 



The nervous system is distinctly developed and pro- 

 vided with several ganglionic centres, in all the molluscous 

 classes from the lowest compound forms of tunicata to 

 the highest of the cephalopods, and notwithstanding the 

 remarkable diversity of form which the animals of this 

 division present, we can trace a certain similarity of cha- 

 racter and unity of plan in the development of this system, 

 and in its typical forms, throughout all the cyclo-gangliated 

 classes. As all molluscous animals are aquatic, excepting 

 a few of the gasteropods, their nervous fibres present the 

 same soft and pellucid character observed in other aquatic 

 invertebrata, which often renders it less easy to trace 

 their ramifications and to detect their plan of distribution, 

 than to follow the denser opaque fibres of the air-breathing 

 tribes, and has repeatedly caused them to be mistaken for 

 sanguiferous or chyliferous vessels. In the short and broad 

 trunks of the animals of this division, as in the round bodies 

 of the radiata, the nervous system is characterised by a 

 tendency to accumulate around the entrance to the alimentary 

 canal, but from the high position of the molluscous classes 

 in the scale, their nervous oesophageal collar is provided 

 with distinct and often numerous ganglia. The same co- 

 lumnar arrangement of the great nervous centres, which I 

 have long observed and described in most of the articulated 

 classes, I have found to exist also in the molluscous, though 

 in a less extended or recti-lineal form. In the tunicated 

 and conchiferous animals the columns are chiefly disposed 

 beneath the alimentary canal; in the gasteropods and the 

 pteropods they are more equally distributed around the 

 entrance to the stomach; and in the more elevated forms of 

 cephalopods they at length mount to that supra-cesophageal 

 position which they preserve in all the vertebrata where 



