ON CLASSIFICATION. 3 



of animals into such as have a simple digestive cavity, receiving 

 and expelling its contents by the same orifice, and such as have 

 an aperture for the expulsion of the contents of the alimentary 

 canal distinct from that by which food is taken into the stomach, 

 is by no means of practical utility, although this circumstance, as 

 we shall afterwards see, has been much insisted upon. 



Hunter's arrangement of the animal kingdom in conformity 

 with the structure of the heart, was a great improvement upon 

 that of Linnseus, founded upon the same basis. He divides in 

 this manner all animals into five groups. 



I. Creatures whose hearts are divided into four cavities Mam- 

 malia and Birds. 



II. Those having a heart consisting of three cavities Rep- 

 tiles and Amphibia.* 



III. Animals possessing a heart with two cavities Fishes 

 and most Mollusca. 



IV. Animals whose heart consists of a single cavity - Articu- 

 lated Animals. 



V. Creatures in which the functions both of stomach and heart 

 are performed by the same organ, as in Medusce. 



We shall pass over Hunter's sketches of arrangements founded 

 on the respiratory and reproductive organs, as offering little satis- 

 factory ; but the researches of this profound physiologist upon the 

 employment of the nervous system for the purpose of zoological 

 distribution, did much to approximate a more natural method of 

 classification, afterwards carried out with important results. 



(5.) The appearance of the " Animal Kingdom distributed in 

 accordance with its organization" of Cuvier, formed a new and im- 

 portant era in Zoology. In this we find all creatures arranged in 

 four great divisions, VERTEBRATA, MOLLUSCA, ARTICULATA, and 

 R ADI AT A. These divisions, with the exception of the first, are 

 named from the external appearance of the creatures composing 

 them, nevertheless the three first are defined by characters exclu- 

 sively drawn from their internal organization, the arrangement of 

 the nervous system being essentially the primary character of dis- 

 tinction, and have been found to be strictly natural ; whilst the 

 last division, characterized by the appellation of R ADI AT A, in the 

 formation of which the structure of the nervous system has 



* For the important discovery that the heart of the Amphibia is divided into three 

 cavities, instead of being composed of a single auricle and ventricle, we are indebted to 

 Professor Owen. Vide Zool. Trans. Vol. I. 



