8 THE COMMON FROG. [chap. 



Sponges, the Infusoria, and all the lower forms of 



animal life. 



Now the whole of these sub-kingdoms may be con- 

 trasted with the last and seventh, which bears the 

 name Vertebrata, from which they all differ in several 

 important particulars, and therefore they are often 

 spoken of by the common and convenient term 

 Invertebrata. 



When we examine a fish (such as a sole, a herring, 

 or a mackerel), one of the first things likely to be 

 noticed by us on dividing it, is a solid structure — the 

 backbone — extending from the head to the tail, and 

 coated externally by the flesh. 



This backbone is soon seen to be made up of a 

 number of pieces jointed together. Each piece is 

 called in natural history a vertebra, and every animal 

 in which such a structure is found, is called, on that 

 account, a Vertebrate animal. 



Now every kind of beast and reptile agrees with 

 these fishes in the possession of the vex'tebrate 

 backbone, as well as in a variety of other important 

 characters, which constitute the definition of the sub- 

 kingdom Vertebrata. 



Thus in the development of the ^gg of every 

 Vertebrate (such e.g. as in that of the fowl), the first 

 indication of the future animal, is the appearance on 

 part of its surface of a minute longitudinal furrow 

 called \.\\^ primitive groove. Next the margins of this 

 groove ascend to meet together above, thus enclosing 

 a canal, the lining of which becomes thickened and 

 transformed into no less important a structure than 

 the brain and spinal marrow. 



