246 ON THE CLASSES OF THE 



culation, also varying in its nature. Their white blood is 

 circulated through veins and arteries, by means of a heart 

 placed, not, as in fisheS; between the veins of the body and 

 the organs of respiration, but between these and the arte- 

 ries. This aortic heart is one of the best characters for 

 grouping the MoUusca together ; and it is the more valu- 

 able, as these animals offer to the Naturalist almost every 

 system possible of generation and digestion. They are 

 furnished with an astonishing variety of organs of masti- 

 cation and deglutition, their stomachs being sometimes 

 simple, sometimes multiplied, and often armed with pecu- 

 liar processes. They often possess salivary glands, and 

 always a considerable liver. Some species have jaws and 

 a tongue, other species neither ; and this variation takes 

 place in animals obviously so near to each other, as to have 

 induced some persons to conclude, that little advantage in 

 the arrangement of these animals is to be derived from the 

 study of their system of nutrition. 



The MoUusca are in general provided with a calcareous 

 covering or shield, bearing no analogy whatever to the shell 

 of coleopterous insects, but serving only for purposes of 

 defence and of shelter to the soft humid skin of its posses- 

 sor. These shells usually bear a strong relation to the dis- 

 position of the organs of circulation and respiration in the 

 animals themselves ; but though doubtless absolutely ne- 

 cessary to be studied, they are to be viewed with great 

 caution in our attempts to arrive at the natural system. 

 For since the organs of respiration and circulation them- 

 selves may lead and indeed often have led naturalists into 

 evident errors of arrangement, so it is to be expected that 

 the form of the inorganic covering, which nature appears 

 to have provided for these parts by concrete exudations 



