THE METAMORPHOSIS. 



the body is produced. In some worms, therefore, we can admit but 

 of two limbs, namely, an anterior one, in which the mouth lies, and 

 which, by the suction of the mouth, affixes itself, and a posterior one, 

 which possesses the sucking cavity, and which, by the help of this 

 organ, can attach itself. In the Annulata, which consist wholly of 

 rings, for instance, the earth worm, small setae supplant the sucking 

 cup ; in the higher Annulata, these setae develope themselves into feet, 

 which remain in the Crustacea, Myriapoda, Arachnodea, and insects ; 

 in the last, organs of flight are superadded. Thus insects maintain, 

 in accordance with the law of successive development, the highest 

 grade among all annulated animals or Arlhrozoa. 



248. 



It therefore appears that, in the further development of the three 

 chief grades of the animal kingdom, the place of abode and the thence 

 proceeding influence of the external world (the external medium) 

 has a very peculiar effect upon the animal organism. There are, 

 however, but three differences of abode, which are the water, the 

 earth, and the air. But in these three chief groups of the animal 

 kingdom, particularly in the second and in the third, we find three 

 groups subordinate to these chief groups, which are determined 

 by the places of abode. Amongst the Vertebrata these groups have 

 long been known as classes ; and are called fishes, as water-verte- 

 brata ; birds, as air-vertebrata ; and mammalia, as earth-vertebrata. 

 To these a fourth class is associated, that of the Amphibia, which 

 apparently is not to be arranged with them, but which, however, 

 presents itself as highly necessary. The living in water, air, and 

 earth are, notwithstanding their great resemblance to each other, so 

 strikingly different, that the animal organism cannot pass directly from 

 one grade to the other, but it requires a connecting member, wherein 

 the organisation is adapted to a residence in both elements, From this 

 transition I have called all such classes classes of transition. 



The group of Arthrozoa admit of being separated in the same 

 manner, if the division may be deduced from the mode of their develop- 

 ment. We obtain thus, therefore, in their four classes : 



1. The WATER-ARTHROZOA. Comprising the intestinal worms 



(Endozoa) and the Annulata. 



2. The CLASS OF TRANSITION. Here stand, as the direct 



links of transition, the wheel animals (Ififusoria rolatoria, 



E E 



