16 MANUAL OF THE MOLLFSCA. 



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

 They produce eggs, and the young generally pass through 

 one preparatory, or lai^val, 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 tho 

 presence of external objects. The nerves by which motion is 

 produced are distinct from these, but so accompany them as to 

 appear 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 will.* 



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

 ring surrounding the throat (^oesophagus), and giving off nerves 

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

 nerves radiate are enlargements termed centres (^ganglia), those 

 on the sides and upper part of the ring represent the brain, and 

 supply nerves to the eyes, tentacles, and mouth ; other centres, 

 connected with the lower side of the oesophageal ring, send 

 nerves to the foot, viscera, and respiratory organ. In the 

 bivalves the branchial centre is the most conspicuous, and is 

 situated on the posterior adductor muscle. In the tunicaries 

 the corresponding nervous centre may be seen between the two 

 orifices in the muscular tunic. This scattered condition of the 

 nervous centres is eminently characteristic of the entire sub- 

 kingdom. 



Organs of special sense. — Siglit. The eyes are two in number, 

 placed on the front or sides of the head ; sometimes they are 

 sessile, in others stalked, or placed on long pedicels {ommatophora). 

 The eyes of the cuttle-fishes resemble those of fishes in their 

 large size and complicated structure. Each consists of a strong 

 fibrous globe {sclerotic), transparent in front {cornea), with the 

 opposite internal surface {retina) covered by a dark pigment 

 which receives the rays of light. This chamber is occupied by 

 an aqueous humour, a ciystalline lens, and a vitreous humour, 

 as in the human eye. In the stromhidce, the eye is not less 

 highly organised, but in most of the gasteropoda it has a more 

 * See " Muller'a Elements of Pliysiology," editetl by Dr. BaJy. 



