ILLUSTRATED NATURAL HISTORY. 
NAUTILUS WITHOUT THE SHELL : DIVISION MOLLUSCA. 
ing tlie animal world, bad imposed upon himself, in the beginning, certain fixed rules, from which 
he would not swerve. 
In this manner we obtain Five Groups or Divisions, each of which leads us a step higher than 
the others — although it is by no means to be supposed that we have here that gradually ascend- 
ing chain of beings so much talked of, in which every species, from the lowest to the highest, is 
supposed to form a link. It is merely in their most highly organized members that the mutual 
superiority or inferiority of these divisions can be recognized ; and, as a general rule, it may be said, 
at all events in respect to the Radiata, Articulata, and Mollusca, that the highest members of each 
group are considerably more perfectly organized than the lower members of the others. The Pro- 
tozoa and Vertebrata appear to be exceptions to this rule ; for the most highly organized of the 
former can scarcely be regarded as superior even to the lowest forms of the other divisions, while 
the fishes, which constitute the lowest members of the vertebrate division, still appear to be more 
highly organized than the lowest Mollusca. 
These five divisions are represented in the following classification, though they are presented in a 
reversed order, it being more according to our habits of observation to begin the study of the animal 
kingdom with the highest classes, and thence to proceed to those of a lower grade, as they follow 
in the places assigned to them. 
In regard to this classification, we may remark that, simplicity being a leading object, we have 
only presented the divisions of Species, Genera, Orders, Classes, and Divisions, leaving out many 
subdivisions of tribes, families, &c., as unnecessary in a table of this nature. In the progress of 
the work we shall have occasion to repeat, and in some instances to enlarge upon, the outline 
given in the preceding pages. 
