ON MOLLUSCS IN GENERAL. 681 



may be compared with that of the vertebrate animals, to which four 

 classes belong, it is not inexpedient, before we pass to the consi- 

 deration of the different classes which have here, with more or less 

 propriety, been adopted, to indicate the common characters which 

 distinguish the molluscs from the rest of the worms. 



The body of these animals is covered by a skin, soft and con- 

 stantly moist, to which the muscles are attached, and in which or on 

 which a calcareous secretion is usually effected. The external in- 

 tegument, which indeed has also been named mantle (although 

 properly a free production on the dorsal surface ought alone to bear 

 that name), encloses as well the intestines as the nervous system 

 also. The central parts of this nervous system consist of ganglia, 

 which either form a ring round the oesophagus or lie more dis- 

 persedly, but not behind each other in a row on the abdominal sur- 

 face, as is the case in the insects. The molluscs ordinarily present a 

 much less similarity of the right and left half of the body than the 

 articulate animals already reviewed by us, or the vertebral animals 

 to be considered in the sequel. Many have no head distinct from 

 the rest of the body. The organs of sense are on the whole slightly 

 developed. In the most composite molluscs however, in the Sepics 

 and other Cephalopods, there are found not only two highly de- 

 veloped eyes, but also rudiments of auditory organs. The move- 

 ments are on the whole creeping and slow. Some, that live in 

 water, are immoveably attached to different objects. Many headless 

 bivalve molluscs have indeed a springing motion, yet this is in a far 

 less degree successive than in articulate animals. 



The inferior degree of development of the organs of animal life 

 is the cause that many writers of the present day, as LINN^US 

 formerly, still place the molluscs lower than the insects in the 

 arrangement of the animal kingdom. 



More perfect than the organs of animal life are those of the vege- 

 tative, those for secretion, nutrition, and propagation. The respi- 

 ratory organs are usually gills. In most molluscs a heart is 

 present, which receives the arterial blood from the organs of respi- 

 ration, and distributes it by arterial tubes to the different parts of 



affinites des animaux auxquels on a donn le nom de Vers. From this memoir read in 

 1795 before the >Sbc. d'Hist. nat. of Paris, it appears that even thus early the class of 

 the molluscs was distinguished by CUVIER and defined as in his later works. 



