128 Selections. [No. 7, new sep.ies. 



ly possible to give any one characteristic distinctive of all. The 

 separate classes indeed are sharply divided, and the whole kingdom 

 is divided into two sub kingdoms which differ so much from each 

 other that they ought rather each to form one kingdom. 



In many Molluscs, the outer appendages of the body, the organs 

 for moving, the feelers, &c, are symmetrically arranged, but this 

 order is far from embracing the whole body, for the interior organs 

 are often most unsymmetrically arranged. The nervous system is 

 divided into two different types. In the Molluscoids we find but 

 one knot of nerves, which stretches out its branches in all direc- 

 tions ; in the Mollusca on the contrary, the nervous system consists 

 of scattered knots, which are strewed rather irregularly over the 

 body, and connected with each other by threads : — the arrangement 

 of these knots is however so varied that we must take them in con- 

 nection with each different order. The organs of the senses too are 

 equally varied, sometimes wanting altogether, sometimes well- 

 developed. Feelers are seldom to be found as the whole surface 

 of the soft body is susceptible of outward impressions, yet there 

 are sometimes lips, or flaps placed round the mouth, sometimes 

 hairs or arms, and sometimes contractile feelers, all of which are 

 evidently for the purpose of feeling. The seeing-apparatus is 

 not developed in all ; in some there is found only one rudimen- 

 tary eye while others possess a great number of incomplete eyes 

 * which are not even placed in the neighbourhood of the head ; — in 

 the higher classes indeed, where there is but one pair of eyes which 

 are placed in the head, they are developed to a very important de- 

 gree. In the Molluscs we meet with hearing organs for the first 

 time : they are round bladders, sometimes possessing a short but 

 important hearing-nerve. Inside these bladders a clear fluid is to be 

 found, and a firm concretion of carbonated lime, which sometimes 

 forms one round hearing stone, sometimes many, which are crys- 

 talline : these seem to be kept in constant tremulous motion by 

 cilise which clothe the inner surface of the bladder. 



The skin of the molluscs is more or less leather-like, generally 

 provided with a slimy slippery covering ; in a few it is glassy clear 

 and transparent; but in the greater number we find shells, which 



