INTEODUCTION. 1 1 



General Divisions op the Animal King com. 



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 InvertehraUi, and " Vertebrate Animals " 

 or Vertehrata. The leading characters which distinguish these 

 two great sections are exhibited in the subjoined diagram, and 

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

 body, if divided transversely or cut in two, shows only 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, Diagrammatic section of one of the higher Invertehrota. B, Section of one 

 of the Vertehrata (slightly altered from Huxley) : a Wall of tlie hody ; 6 Alimentary 

 canal ; c Circulatory system ; n Nervous system ; n' Brain and spinal cord of the 

 Vertehrata, enclosed in a separate tube ; ch Notochord or chorda dorsalis. 



the body ; and a nervous system, by 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- 

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

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

 the deposition in it of horny matter, of lime, or of flint. The limbs, 

 when present, are turned towards th^t side of the body upon which 

 the main portions of the nervous system are situated. In the Ver- 

 tehrata, 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 spinal cord). In the other 

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

 system, and certain other portions of the nervous system, which ai'e 



