INVERTEBRAL ANIMALS. 3 



tre. In this last division, too, the lowest in the scale of animated 

 beings, the nervous and circulating system almost disappears ; 

 the respiratory apparatus is almost always on the surface of the 

 body ; and in the greater number the intestinal canal presents 

 the appearance of a simple sac without outlet. The last fami- 

 lies of this division present the appearance of a homogeneous 

 pulp, indistinctly perceived to possess animal life from giving 

 indications of motion and sensation. 



This general arrangement of Cuvier, founded upon the com- 

 parative organization of the animal kingdom in a descending 

 scale, from those in which all the animal functions exist in their 

 most perfect state, to those in which vitality is but indistinctly 

 exhibited, has been followed by most of the recent writers, as 

 at once philosophical and natural. Of the subsidiary divisions, 

 however, some modifications have been proposed by authors 

 with whom these divisions have been more an object of particular 

 study ; and in the following pages, while we adhere generally 

 to the great outline given by the first naturalist of the age, we 

 shall adopt the details from the other authors who have filled 

 up this outline. In the Mollusca and Radiata, therefore, the 

 arrangement of Lamarck, as given in his well known work, the 

 Histoire Naturelle des Animauos sans Vertebres 9 will be chief- 

 ly followed ; and in the Crustacea and Insecta, that of the 

 celebrated entomologist, M. Latreille. The classes of the Inver- 

 tebrata will therefore be treated in the following order : 



DIVISION II. MOLLUSCA. 



CLASS I. MOLLUSCA, CLASS III. TUNICATA, 



II. CONCHIFERA, IV. CIRRHIPEDA. 



DIVISION III. ARTICULATA. 



CLASS V. ANNELIDES, CLASS VIII. MYRIAPODA, 



VI. CRUSTACEA, IX. INSECTA. 



VII. ARACHNIDES, 



DIVISION IV. RADIATA. 



CLASS X. ECHINODERMA, CLASS XIII. POLYPI, 



XI. ENTOZOA, XIV. INFUSORIA. 



XII. ACALEPHA, 



