335 



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 fibrine 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 sohd levers, the 

 Mollusca cannot make rapid springs. 



The irritability of the greater number of the Mollusca is very great, and is retained 



• In the oriinnal, there is here a lun^ note, containintj an expo- I t From this mode of expression, wc infer that Cuvier had adopted 

 sition of tiic LinuKan classilication of avertebrated animals, and the theory, that the brain and spinal cord are ttic result of a union of 

 also the modification of it proposed by Brogui^res. 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 I opposite opinion was that maintained by Haller, and all the earlier 

 \V^o. — Ed. j physiolot^ists, — Kd. 



