INTEODUCTION. 



11 



General Divisions or the Animal Kinciiom. 



Acting upon the principles above indicated, and by a careful com- 

 parison of the morphological and physiological differences between 

 different animals, naturalists have divided the entire animal king- 

 dom into a number of divisions, the general arrangement of which 

 may be very briefly indicated. Firstly, the whole animal kingdom 

 may be divided into two great sections, known respectively as "In- 

 vertebrate Animals" or Invertehrata, and "Verteljrate Animals" 

 or Vertebrata. The leading characters which distinguish these 

 two great sections are exhibited in the sul>joined diagram, and 

 may be shortly stated as follows : In all Invertebrate animals, the 

 body, if divided transversely or cut in two, shows oidy a single tube 

 containing all the vital organs (fig, 2, A). These organs in the 

 higher Invertehrata consist of a digestive or alimentary tube ; a cir- 

 culatory system, by which the vital fluids are distributed through 



Fig. 2.— A, DiagraniinatiP: section of one of tlie liiglier fuvfytidnvta. B, Section of one 

 of tlie VertebroM (slightly altei-ed from Huxley): a Wall of tlie IioUy; b Alinienttiiy 

 canal; c Circuljitory system; n Nervous system; n' Brain and sjiinal cord oftlje 

 Vertebrata, enclosed in a separate tube; ch Notocliord or chorda dorsalis. 



the bod}'; and a nervous system, l:iy which the animal is brought 

 into relation with the outer world. Any or all of these vital 

 organs, however, may be wanting, or may be imperfect or rudi- 

 mentaiy. When there is any skeleton, this is usually external, and 

 is mostly nothing more than a hardening of the skin, produced by 

 the depjosition in it of horny matter, of lime, or of flint. Tlie limbs, 

 when present, are turned towards that side of the body upon which 

 the main portions of the nervous system ai'e situated. In the Ver- 

 tebrata, on the other hand, the body, if transversely divided, ex- 

 hibits two tubes (fig. 2, B). In the one is placed the main mass of 

 the nervous system (the brain and sjjinal cold). In tlie other 

 tube are the alimentary canal, the main organs of the circulatory 

 system, and certain other jiortioiis of the nervous system, which are 



