2 VERTEBRATES. 
common characteristics. It is among these lower animals that the want of a true 
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 
radiated or star-like form of the body, so well exhibited in the Star-fishes and the Sea- 
anemones. Their neryous system is very obscure, and in many instances so sheht as to 
baffle even the microscope. Many of the Radiates possess the faculty of giving out a 
phosphorescent light, and it is to these animals that the well-known luminosity of the 
sea is chiefly owing. 
5th. The Prorozoa, or primitive animals, are, as far as we know, devoid of internal 
organs or external limbs, and in many of them the signs of life are so feeble, that they 
can scarcely be distinguished from vegetable germs. The Sponges and Infusorial Animal- 
cules are familar examples of this division. 
VERTEBRATES. 
The term Vertebrate is derived from the Latin word vertere, signifying to turn; and 
the various bones that are gathered round and defend the spinal cord are named vertebree, 
because they are capable of being moved upon each other in order to permit the animal 
to flex its body. Were the spinal cord to be defended by one long bone, the result would 
be that the entire trunk of the animal would be stiff, graceless, and exceedingly liable to 
injury from any sudden shock. In order, therefore, to give the body latitude of motion, 
and at the same time to afford effectual protection to the delicate nerve-cord, ou which 
the welfare of the entire structure depends, the bony spine is composed of a series of 
distinct pieces, varying in form and number according to the species of animal, each 
being affixed to its neighbour in such a manner as to permit the movement of one upon 
the other. The methods by which these vertebrae are connected with each other vary 
according to the amount of flexibility required by the animal of which they form a part. 
For example, the heavy elephant would find himself prostrate on the ground if his spine 
were composed of vertebrae as flexible as those of the snakes ; while the snake, if its spine 
were stiff as that of the elephant, would be unable to move from the spot where it 
happened to lie. But in all animals there is some power of movement in the spinal 
column, although in many creatures it is very trifling. 
Anatomy shows us that, in point of fact, the essential skeleton is composed of 
vertebrae, and that even the head is formed by the development of these wonderful bones. 
The limbs can but be considered as appendages, and in many Vertebrated animals, sueh 
as the common snake of our fields, the lamprey, and others, there are no true limbs 
at all. 
The perfect VERTEBRA consists of three principal portions. Firstly, there is a solid, 
bony mass, called the centre, which is the basis of the whole vertebra. From this centre 
springs an arch of bone, through which runs the spinal cord, and directly opposite to this 
arch a second arch springs, forming the guardian of the chief blood-vessel of the body. 
Each arch is called by a name significative of its use; those through which the spinal 
cord runs being termed the neural, or nerve arch, and that for the passage of the blood- 
vessel is named the heemal, or blood arch, There are other portions of the vertebree which 
are developed into the bones, called “ processes,” some of which we can feel by placing a 
hand on any part of the spine. 
It will be seen that, strictly speaking, the vertebree are not of so much importance in 
the animal as the spinal cord, of which the vertebree are but euardians, and that the 
division should rather have been defined by the character of the nerve than by that of the 
bone which is built around it. 
Indeed, wherever the chief nervous column lies, it seems to gather the bony particles, 
and to arrange them round itself as its clothing or armour, This may be seen in a very 
young chicken, if the egg in which it is formed is opened during the first few days of 
incubation. 
