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SECOND GREAT DIVISION OF THE ANIMAL KINGDOM. 
THE MOLLUSCA.* 
The Mollusca have no articulated skeleton nor vertebral canal. Their nervous 
system does not unite in a spinal cordf, but merely in a certain number of medullary 
masses dispersed in different points of the body, the principal one of which, called the 
brain, is placed crosswise upon the gullet, encircling it with a nervous collar. Their 
organs of motion and of the senses have not the same uniformity in number and 
position as in the Vertebrated Animals ; and the variety is still more striking with the 
viscera, particularly in relation to the position of the heart and respiratory organs, and 
even in the structure and nature of the latter ; for some Mollusca breathe the free air, 
and others the fresh or salt water. In general, however, their external organs, and 
those of locomotion, are symmetrical on the opposite sides of a middle axis. 
The circulation of the Mollusca is always double, — that is to say, their pulmonary 
circulation always makes a separate and complete circuit ; and this function is always 
aided by one fleshy ventricle at least, placed, not as in the Fishes, between the veins 
of the body and the arteries of the lung, but, on the contrary, between the veins of the 
lung and the arteries of the body. It is, consequently, an aortic ventricle. The 
family of Cephalopods alone is provided, besides, with a pulmonary ventricle, which is 
even divided into two. The aortic ventricle is also divided in some genera, of which the 
Area and Lingula are examples : at other times, as in the remaining bivalves, its auricle 
only is divided. 
When there is more than one ventricle, they are not united together to form a single 
organ, as in animals with warm blood, but they are often placed considerably apart, so 
that we may say that then there are several hearts. 
The blood of the Mollusca is white, or bluish ; and the flbrine appears to be pro- 
portionally less abundant than in the blood of Vertebrated Animals. There is reason to 
believe that their veins perform the functions of absorbent vessels. 
Their muscles are attached to different points of their skin, and form there tissues 
more or less complicated and close in texture. The motions of these tissues are limited 
to contractions in different directions, which produce inflexions and prolongations, or 
relaxations, of their different parts; by means of which the creatures creep, swim, and 
seize upon various objects, according as the forms of the parts are adapted to these 
movements ; but as their members are not sustained by jointed and solid levers, the 
Mollusca cannot make rapid springs. 
The irritability of the greater number of the Mollusca is very great, and is retained 
* In the original, there is here a long note, containing an expo- I f From this mode of expression, we infer that Cuvier had adopted 
sition of the Linnsean classification of avertebrated animals, and I the theory, that the brain and spinal cord are the result of a union of 
also the modification of it proposed by Bruguiferes. Cuvier’s first I the nerves, trending from the circumference to certain centres. The 
sketch of the arrangement now to be explained was made in May opposite opinion was that maintained by Haller, and all the earlier 
1795 . — Ed. 1 physiologists. — Ed. 
