XXIV INTRODUCTION. 



certain characters. The most general or comprehensive of 

 these divisions are termed Kingdoms, Sub-Kingdoms, &c. 

 These, again, are divided into Classes, Orders, Families, or 

 Tribes ; and these, again, into Genera or little Families, 

 consisting of one or more Species, that is, of animals essen- 

 tially alike in themselves and their progeny. The lowest 

 division that can be made is into Varieties, Races, or Breeds, 

 which consist of animals agreeing in the characters which 

 we term specific, but differing in certain minor characters, 

 assumed to be the result of known agencies, as climate, 

 temperature, and food. The classifications most commonly 

 received are founded upon that of the illustrious Cuvier, who 

 divided the whole animal kingdom into four great groups, 

 namely, 1. Radiata, or Radiated Animals ; 2. Articulata, or 

 Jointed Animals ; 3. Mollusca, or Soft Animals ; 4. Verte- 

 brata, or Animals having the basis of the nervous system 

 enclosed in vertebrae, or hollow bones. 



The Radiata are so named from the general tendency of 

 their organs to proceed like radii from a common centre. 

 They may be regarded as the simplest in their forms of ani- 

 mated creatures. The nervous system, which, in the higher 

 order of animals., is developed in ganglia and a brain, is in 

 them rudimental, visible, when it can be discovered at all, in 

 a few fibres, surrounding the entrance of the alimentary 

 canal. Many of them present the appearance of a simple 

 digestive sac or tube, furnished with little arms or tenta- 

 cula, or with mouths for fixing themselves to the substances 

 on which they feed. Many of them are so small as to be 

 invisible to the unassisted eye, nay, some so inconceivably 

 minute, that a million of millions, it has been calculated, might 

 be comprehended within the space of a cubic inch. The spe- 

 cies, however, present a vast variety of size as well as of 

 form, from the simplest of all, to those in which new organs 

 are developed, suited to more varied functions. They are 

 all the inhabitants of water, and almost all those whose ha- 

 bits can be observed are predaceous, seizing their prey by 



