2 VERTEBRATES. 



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 residt would be that the entire 

 trunk of the animal wovdd 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 con- 

 nected 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 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 rims 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 incu- 

 bation. 



The position of the spinal cord is always along the back in every Vertebrate animal. The 

 insects, the lobster, and other invertebrate animals exhibit the principal nerve-cords running 

 along the abdomen ; the position, therefore, of the chief nervous cord settles the division to 

 which the animal belongs. This rule is of great importance in classification, because in every 

 group of animals there are some in whom the distinguishing characteristics are so slight tliat 

 they hardly afford a real criterion by which to judge. In the lower divisions the number of 



