SYSTEMS OF HUNTER AND CUYIER. 3 



founded upon the same basis. He arranges in this manner all animals 

 in five groups. 



I. Creatures whose hearts are divided into four cavities Mammalia 

 and Birds. 



II. Those having a heart consisting of three cavities Reptiles and 

 Amphibia*. 



III. Animals possessing a heart with two cavities Fishes and most 

 Mollusca. 



IY. Animals whose heart consists of a single cavity Articulated 

 Animals. 



Y. 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 that is satis- 

 factory; but the researches of this profound physiologist upon the 

 employment of the nervous system for the purpose of zoological distri- 

 bution did much to inaugurate a more natural method of classification, 

 afterwards carried out with important results. 



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

 cordance with its organization," of Cuvier, formed a new and important 

 era in Zoology. In this we find all creatures arranged in four great 

 divisions, YEETEBEATA, MOLLUSCA, AETICULATA, and RADIATA. 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 exclusively drawn from their internal 

 organization, the arrangement of the nervous system being essentially 

 the primary character of distinction, and have been found to be strictly 

 natural ; whilst the last division, characterized by the appellation of 

 EADIATA, in the formation of which the structure of the nervous system 

 has been allowed to give place in importance to other characters of 

 secondary weight, obviously embraces creatures of very dissimilar and 

 incongruous formation. 



(6.) The YEETEBEATA are distinguished by the possession of an in- 

 ternal nervous centre or axis, composed of the brain and spinal cord, 

 which is enclosed in an osseous or cartilaginous case, and placed in the 

 median plane of the body, giving off symmetrical nerves, which are dis- 

 tributed to all parts of the system.. This general definition indicates 

 a large division of the animal world, which, by secondary characters 

 drawn from the structure of their organs of respiration and circulation, 

 is separable into mammals, birds, reptiles, amphibia, and fishes. 



(7.) The MOLLTJSCA have a nervous system constructed upon a very 

 different type, and do not possess any vertebral column or articulated 



* 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.). 



B2 



