520 HISTORY OF CONCHOLOGY. 



difications, founded on development or addition of certain 

 parts, which do not change the essence of the plan." Of 

 these forms the Mollusca furnish the second, of which the 

 essential character is derived from the peculiar arrangement 

 of the nervous system, consisting of some ganglions scat- 

 tered, as it were, irregularly through the body, and from 

 each of which nerves radiate to its various organs. As there 

 is no skeleton, so the muscles are attached to the skin, which 

 forms a soft contractile envelope protected, in many species, 

 by a shell. The greater number possess the senses of taste 

 and sight, but the last is often wanting. " Only one family 

 can boast of the organ of hearing ; they have always a com- 

 plete system of circulation, and organs peculiarly adapted to 

 respiration ; those of digestion and secretion are nearly as 

 complicated as the same organs in vertebrated animals." * 

 The sub-kingdom, characterized and limited by these im- 

 portant features, is next divided into six classes, the charac- 

 ters of which are mostly derived from the organs of locomotion 

 or others not less influential. Thus the Cephalopods bear 

 their feet and arms like a coronet round the summit of the 

 head ; the Pteropods swim in their native seas by fin-like 

 oars ; and the Gasteropods crawl on the belly by means of a 

 flat disk or sole. Reaching now tribes among whom the 

 organs of motion are less developed, and accordingly less 

 influential on their manners, Cuvier resorts to others. Thus 

 the fourth class is named Acephales, because it is strikingly 

 distinguished by the want of head and amorphous form of its 

 constituents ; the Brachiopods are equally acephalous, but 

 near the mouth they have two fringed fleshy organs which 

 simulate feet ; and the Cirrhopods have several pairs of sub- 

 articulated fringed feet, in addition to a multivalvecl shell of 

 a peculiar construction. The orders of these classes, when 

 the class admits of further subdivision, rest upon distinct 

 differences in the structure and position of the branchiae or 

 respiratory organs ; and when we reflect a moment on the 

 paramount necessity of these to the animal, and their neces- 

 sary co-adaptation to its locality and wants, it is scarcely 

 possible to conceive that a happier choice could have been 

 made. 



We have already explained at sufficient length the Cuvier- 

 ian system ; and enough has been now said to show its vast 

 superiority to all that had preceded it. The solidity of its 

 basis is proved by the fact that the numerous recent dis- 

 coveries in this department have not shaken it, or altered its 

 principles. The lower divisions and sections have been im- 



* Memoirs of Cuvier by Mrs. Lee, p. 107 — 109. 



