SECOND GRAND DIVISION 
OF THE 
ANIMAL KINGDOM. 
ANIMALIA MOLILUSCA. 
THE mollusca have neither an articulated skeleton, nor aver- 
tebral canal. Their nervous system is not united in a spinal 
marrow, but merely in a certain number of medullary masses, 
dispersed in different parts of the body, the chief of which, 
termed the brain, is situated transversely on the cesophagus, 
and envelopes it with a nervous collar. Their organs of 
motion and sensation have not the same uniformity of number 
and position as in the vertebrata, and the irregularity is still 
more striking in the viscera, particularly as respects the posi- 
tion of the heart and respiratory organs, and even as regards 
the structure of the latter; for some of these respire elastic 
air, and others salt or fresh water. ‘Their external organs, 
however, and those of locomotion, are generally arranged 
symmetrically on the two sides of an axis. 
The circulation of the mollusca is always double ; that is, 
their pulmonary circulation describes a separate and distinct 
circle. This function is at least always aided by a fleshy 
ventricle, situated between the veins of the lungs and the 
arteries of the body, and not, as in fishes, between the veins of 
the body and the arteries of the lungs. It is then an aortic 
ventricle. The cephalopoda alone are provided with a pul- 
monary ventricle, which is even divided into two. The 
VOL. XII. B 
