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 imdicate the common characters which 
distinguish the molluses 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 cesophagus or lie more dis- 
persedly, but not behind each other im 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 Sepie 
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 immoyeably 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 Linnaus 
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 
afinités des animaux auaquels on a donné le nom de Vers. From this memoir read in 
1795 before the Soc. 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. 
