i VERTEBRATES. 



any 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 nervous system is very obscure, and in many instances so slight 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 PROTOZOA, 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 

 Animalcules are familiar 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 

 vertebrae, 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, on 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 neighbor 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, 

 such 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 oppo- 

 site 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 haemal, or blood arch. There are other 

 portions of the vertebrae 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 vertebrae are not of so much importance in 

 the animal as the spinal cord, of which the vertebrae are but guardians, 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 armor. 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. 



