CLASSIFICATION OF ANIMALS. 77 
swiftness, apparently gifted with sagacity, for they readily elude each other 
in the active dance they keepup..... Increase the power of your glasses, 
and you will soon perceive, inhabiting the same drop, other animals, com- 
pared to which the former were elephantine in their dimensions, equally 
vivacious and equally gifted. Exhaust the art of the optician, strain 
your eyes to the utmost, until the aching sense refuses to perceive the 
little quivering movement that reveals the presence of life, and you will 
-find that you have not exhausted nature.’ 
Classification of Animals. 
The number of different kinds of animals is very great, vastly exceeding 
that of plants; and the diversities amongst them are very wide, not 
merely in size, but in structure and habits. Classification, therefore, 
is absolutely necessary, in order to obtain an intelligent view of the 
animal kingdom; but there are some groups which at once present 
themselves to the observer as distinctly marked, and which therefore 
have been universally recognised from the earliest times. Such is the 
great group of Birds, or that of Fishes; although observation more exact 
was necessary to exclude bats from the former, and whales from the latter. 
The greater group of Vertebrate Animals, in which both Birds and Fishes 
are included, is also so natural that it must have a distinct, place in every 
system of classification. We cannot here give an account of the various 
systems of classification which have been proposed. That of Cuvier 
has been generally received since the beginning of the present century. 
Cuvier regarded the animal kingdom as consisting of four great divisions 
—Vertebrata, Mollusca, Articulata, and Radiata. The first division, Verte- 
brata or Vertebrate Animals, consists of animals having an internal bony 
skeleton, the principal part of which is a back-bone or vertebral column, 
composed of short joints, to which all the other parts are attached. All 
the highest classes of animals belong to this division—Mammals, Birds, 
Reptiles, and Fishes. Man himself, considered as to his corporeal frame, 
belongs to it. All the other divisions of the animal kingdom are united 
under the common designation of Invertebrata or Invertebrate Animals; 
but they differ far more widely from each other than the classes of verte- 
brate animals, and are therefore regarded by Cuvier as forming three great 
groups, each equal in rank with the Vertebrata. His second division, 
Mollusca,! consists of animals which have no skeleton, external or internal, 
Their bodies are always soft and covered with a skin, and often protected 
by an external shell, which they secrete. Snails, slugs, oysters, mussels, 
cockles, and most of the other creatures popularly called shell-fish, belong 
to this division. Cuvier’s third division, Articulata? or Articulated 
1 From Latin molluscus, soft. 2 From Latin articulatus, jointed, 
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