351 



CHAPTER XVIII. 



HETEROGANGLIATA* (Owen) ; MOLLUSCA. (Cuv.) 



(387.) THE term MOLLUSCA, employed by Cuvierto designate 

 the fourth grand division of the animal world, is obviously derived 

 from a very unimportant circumstance of their organization, which 

 the tribes included in it possess in common with innumerable forms 

 both of Acrite and Nematoneurose beings, whose soft bodies are 

 unsupported by any internal or tegumentary framework of sufficient 

 density to merit the name of a skeleton. Subsequent anatomists 

 have therefore, however unwillingly, been compelled to substitute 

 another name for that given by the illustrious French zoologist to 

 this extensive class, the boundaries and relations of which, as at 

 present admitted, remaining precisely as they were first established 

 by his patient and unwearied investigations relative to the anatomi- 

 cal structure of the animals comprised within its limits. 



It is to the arrangement of the nervous system that we must 

 again have recourse in order to discover a distinctive appella- 

 tion ; nor in this shall we be disappointed, for here we at once find 

 a character peculiar to this great section of animated nature, and 

 generally applicable to the various classes composing it. All the 

 Mollusca present nervous ganglia, which, in the more highly organ- 

 ized forms, attain considerable developement and consequent per- 

 fection ; but these nervous centres, instead of being arranged in a 

 longitudinal series of symmetrical pairs, are variously distributed in 

 different parts of the body ; an arrangement exactly correspondent to 

 the want of symmetry observable both in the external configuration 

 of these creatures, and in the anatomical disposition of their internal 

 viscera. Still, however, one large ganglionic mass occupies a posi- 

 tion above the oesophagus, and it is with this that the nerves of the 

 existing senses invariably communicate ; so that we are naturally 

 induced to regard this as the sentient brain, corresponding with the 

 supra-eesophageal ganglion of the ABTICULATA both in position 

 and office. The other ganglia vary considerably both in number 

 and in situation, but, wherever placed, they all communicate with 

 the supra-cesophageal mass ; while the branches derived from them 

 are distributed to the viscera, or to the locomotive organs. 



(388.) Various are the forms, and widely different the relative 



s, dissimilar ; yyyX/v, a ganglion. 



