STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 21 



Chapter IV. 

 STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 



MoLLUSCODS animals possess a distinct nervous system, in- 

 straments appropriated to the five senses, and muscles by which 

 they execute a variety of movements. They have organs, by 

 which food is procured and digested, — a heart, with arteries 

 and veins, through which their colourless fluids circulate, — a 

 breathing-organ, — and in most instances, a protecting shell. 

 Tliey produce eggs ; and the young generally pass through one 

 preparatory, or larval, stage. 



The nervous system, upon which sensation and the exercise 

 of muscular motion depend, consists of a brain or principal 

 centre, and of various nerves possessing distinct properties : the 

 optic nerves are only sensible of light and colours ; the auditory 

 nerves convey impressions of sound ; the olfactory, of odours ; 

 the gustatory, of flavours ; whilst the nerves of touch or feeling 

 are widely diffused, and indicate in a more general way the pre- 

 sence of external objects. The nerves by which motion is pro- 

 duced, are distinct from these, but so accompany them as to ap- 

 pear like parts of the same cords. Both kinds of nerves cease 

 to act when their connection with the centre is interrupted or 

 destroyed. There is reason to believe, that most of the move- 

 ments of the lower animals result from the reflection of external 

 stimulants (like the process of breathing in man), without the 

 intervention of the wiU.* 



In the mollusca, the principal part of the nervous system is 

 a ring surrounding the thi'oat (oesophagus), and giving off nerves 

 to different parts of the body. The points from which the nerves 

 radiate, are enlargements, termed centres (ganglia), those ou the 



* See Miiller's Elements of Philosophy, edited by Dr. Baly. 



