MOLLUSCA. 3 
ever, first indicate those which they possess in 
common, and by which they are naturally grouped 
together. 
The nervous system demands our first attention. 
Instead of a great mass of nervous substance accu- 
mulated in one place, and a lengthened spinal cord 
_ proceeding from it, giving out threads to all parts 
of the body, as in the VERTEBRATA, we find the 
nervous centres numerous, unsymmetrical and dis- 
posed in various parts of the system, no one having 
so decided a predominance over the others in bulk, 
as to merit the appellation of a brain. There is, 
however, one mass larger than the rest, which is 
always placed either above the gullet (wsophagqus), 
or encircling it, in the form of a thickened ring; 
and from this the nerves that supply the organs of 
sense invariably originate. This mass, or ganglion, 
must undoubtedly be regarded as the representative 
of the brain; for in the most highly organized 
animals of the Division, the Squids and Cuttles 
(Cephalopoda), this encirling mass is enclosed and 
defended by a case of cartilage, the lingering rudi- 
ment of a bony skull. 
The accompanying engraving, which is copied 
from Professor Grant’s “ Outlines of Comparative 
Anatomy,’ will give the reader an idea of the 
system of nerves and ganglia, with some of the 
other organs, as they appear in bulla liguaria, a 
large and handsome shelled Mollusk found on the 
British coasts. 
In the above figure the chief ganglion forms a 
ring, (marked ee,) ; anterior to this there is a small 
ganglion, not seen, because situated below the bulb 
of the gullet (a), just before the insertions of its 
diverging muscular bands (c), and behind the 
