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INTRODUCTION. 
N order to understand any science rightly, it needs that the student should proceed 
to its contemplation in an orderly manner, arranging in his mind the various 
portions of which it is composed, and endeavoring, as far as possible, to follow 
that classification which best accords with nature. The result of any infringement 
of this rule is always a confusion of ideas, which is sure to lead to misappre- 
hension. So, in the study of living beings, it is necessary to adhere to some 
determinate order, or the mind becomes bewildered among the countless myriads 
of living creatures that fill earth, air, and water. 
That some determinate order exists is evident to any thinking mind, but the discovery of 
the principle on which this order is founded is a problem that as yet has received but a partial 
solution. We already know some of the links of that wondrous chain that connects Man with 
the microscopic animalcule, but the one plan on which the Animal Kingdom is formed, has 
yet to be made known. 
It is impossible to contemplate the vast mass of animal life without the conviction that the 
most supreme harmony has been observed in their creation, and the most perfect order exists 
in their connection one with the other. Whatever may be the key to this enigma, — and it is 
of a certainty a very simple one, possibly eluding us from its very simplicity — from the days 
of Aristotle to the present time zoologists have been diligently seeking for the true system of 
animated nature ; and until that auspicious discovery be achieved, we must be content with 
making as near an approximation as possible. 
As a general arranges his army into its greater divisions, and each division into regiments 
and companies, so does the naturalist separate the host of living beings into greater and smaller 
groups. The present state of zoological science gives five as the number of divisions of which 
the animal kingdom is composed, the highest of which is that in which Man himself is, by 
some, placed. These are called Vertebrates, Molluscs, Articulates, Radiates, and Protozoa. 
Of each of these divisions a slight description will be given, and each will be considered more 
at length in its own place. 
1st. The Vertebrates include Man and all the Mammalia, the Birds, the Reptiles, and 
the Fish. 
The term Vertebrate is applied to them because they are furnished with a succession of 
bones called “vertebrse,” running along the body and forming a support and protection to the 
nervous cord that connects the body with the brain by means of numerous branches. The 
Vertebrates, with one or two known exceptions, have red blood and a muscular heart. 
2d. The Mollusca, or soft-bodied animals, include the Cuttle-fish, the Snails, Slugs, 
Mussels, &c. Some of them possess shells, while others are entirely destitute of such defence. 
Their nervous system is arranged on a different plan from that of the Vertebrates. They have 
no definite brain, and no real spinal cord, but their nerves issue from certain masses of nervous 
substance technically called ganglia. 
3d. The Articulates, or jointed animals, form an enormously large division, comprising 
the Crustaceans, such as the Crabs and Lobsters, the Insects, Spiders, Worms, and very 
many creatures so different from each other, that it is scarcely possible to find any common 
characteristics. It is among these lower animals that the want of a tine classification is most 
severely felt, and the present arrangement can only be considered as provisional. 
4th. The next division, that of the Radiated animals, is so named on account of the 
