462 HETEROGANGLIATA. 



CHAPTER XVII. 



HETEKOGANGLIATA* (Owen). 

 MOLLUSCA (Cuvier). 



(1191.) THE term MOLLUSCA, employed by Cuvier to 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 of very 

 dissimilar 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 unwil- 

 lingly, been compelled to substitute another name for that given by 

 the illustrious French zoologist to this extensive class, the bounda- 

 ries and relations of which, as at present admitted, remain precisely as 

 they were first established by his patient and unwearied investigations 

 relative to the anatomical structure of the animals comprised within its 

 limits. 



(1192.) It is to the arrangement of the nervous system that we must 

 again have recourse in order to discover a distinctive appellation ; 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 appli- 

 cable to the various classes composing it. All the Mollusca present 

 nervous ganglia, which, in the more highly organized forms, attain con- 

 siderable development and consequent perfection; but these nervous 

 centres, instead of being arranged in a longitudinal series of symme- 

 trical pairs, are variously distributed in diiferent parts of the body, 

 an arrangement exactly correspondent to the want of symmetry ob- 

 servable 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 position above the oesophagus, and it 

 is with this that the nerves of the existing senses invariably communi- 

 cate ; so that we are naturally induced to regard this as the sentient 

 brain, corresponding with the supra- cesophageal ganglion of the AETI- 

 CULATA both in position and office. The other ganglia vary considerably 

 both in number and in situation ; but, wherever placed, they all com- 

 municate with the supra-oesophageal mass, while the branches derived 

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



(1193.) Various are the forms and widely different the relative per- 

 fection of the Mollusca, as regards their endowments and capabilities. 

 Some, as the POLYZOA, fixed to the surface of foreign bodies, either 

 immoveably or by the intervention of a flexible pedicle, entirely deprived 

 * erepos, dissimilar ; yayyXt'ov, a ganglion. 



