PRELnnKARY OBSERVATIONS. 11 



It is to some interesting portions of structure 

 presented by the animals of these four great 

 groups or classes that we shall chiefly attend. 

 They constitute, according to the arrangement 

 of modern naturalists, a sub-kingdom of the 

 animal worldj termed by Cuvier the vertebrate ; 

 and in like manner the classes of animals still 

 lower in the scale are grouped into great sub- 

 kingdoms ; but the grounds of distinction be- 

 tween those classes are, in many instances, less 

 rigidly marked out. Their characters are, in 

 fact, more indeterminate. 



The sub-kingdom succeeding the vertebrate, 

 (that is, possessing a true brain and spine.) 

 comprehends numerous classes in which nerves 

 exist, but in which there is no true brain, (in 

 some, perhaps, rudiments of it,) but no spinal 

 cord or spinal column. These have been 

 commonly called molluscous animals, including 

 the cuttlefishes, the nautilus, the little northern 

 clio the food of the whale ; slugs and snails, or 

 univalve-sheUed slugs, terrestrial and aquatic, 

 as the whelk, the murex, etc. ; bivalve-shells 

 or shellfish, as oysters, mussels, etc.; certain 

 mollusks invested in a sort of cartilaginous 

 tunic, found in the warmer seas ; including the 

 pyrosoma, celebrated for its phosphorescence, 



