16 MANUAL OF THE MOLLUSCA. 
breathing-organ; and, in most instances, a protecting shell. 
They 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 
neryes convey impressions of sound; the olfactory, of odours ; 
the gustatory, of flayours; whilst the nerves of touch or feeling 
are widely diffused, and indicate in a more general way the 
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 (esophagus), and giving off nerves 
to different parts of the body. The points from which the 
neryes radiate are enlargements termed centres (ganglia), those 
on the sides and upper part of the ring represent the brain, and 
supply neryes to the eyes, tentacles, and mouth; other centres, 
connected with the lower side of the csophageal ring, send 
neryes 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 
neryous centres is eminently characteristic of the entire sub- 
kingdom. 
Organs of special sense.—Sight. 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 crystalline lens, and a vitreous humour, 
as in the human eye. In the strombide, the eye is not less 
highly organised, but in most of the gasteropoda it has a more 
* See “Miiller’s Elements of Physiology,” edited by Dr. Baly. 
