272 THE EVOLUTION OF THE METAZOA 



secondarily simplified Turbellaria. The only objection to this 

 that could be mentioned in this connection is the small size 

 ot the Acoela. There is a generally valid rule that a general 

 decrease of an animal body, when it passes certain limits, 

 not infrequently leads to an extensive reduction of the animal 

 organization which appears as its simplification. It can lead 

 not only to an abolition of the system of blood vessels and 

 of the respiratory organs (numerous instances in the very 

 small species of the Acarina) but also of the whole feeding and 

 digestive systems (the dwarfed males of numerous Rotatoria). 

 The conditions are clear in these and in many other cases 

 because here the causal relationship between the diminution 

 of the size of the animal and the simplification of the organizat- 

 ion can easily be identified. In all these cases organs and systems 

 of organs can be found in certain forms while the same cannot 

 be found in the related forms whose size has been secondarily 

 diminished. In the case of the dwarfed male Rotatoria we find 

 even the female animals that belong to the same form equipped 

 with such organs; these female forms have at the same time 

 also a considerably larger size. 



Somewhat less easy to survey are those cases w^here a larger 

 animal group consists exclusively of very small species, which 

 at the same time also have a simpler structure, this can be 

 observed, for example, in the Rotatoria and in the Tardigrada. 

 A detailed study of these groups, however, shows that there 

 are some facts which prove with great certainty that the size of 

 the species, which belong to these groups, had been secondarily 

 diminished and that a simplification of their organization had 

 taken place as a consequence of such a diminution. One of the 

 elements that can be helpful in this connection is the trend 

 towards a decreased number of cells and their constancy; 

 another element is the loss of ability to regenerate (in Rotatoria 

 the lost parts of cells only are able to regenerate). Thirdly, we 

 can tale into consideration a highly developed ecologic 

 specialization, e.g. the adaptation to small recesses in which 

 an animal lives. 



