392 THE EVOLUTION OF THE METAZOA 



best seen if we juxtapose, for example, on the one hand an 

 acoelan or a hydropolyp, the males of various Rotatoria, the 

 Cestoda, and the solitary Endoprocta, and on the other hand 

 the Cephalopoda, the various higher Turbellaria, the Nemato- 

 morpha, etc. 



It must be admitted, however, that certain very conspicuous 

 •common traits of the general organization and of the level 

 of this organization can be found in all these animal groups 

 which are otherwise so widely' different. This proves that they 

 do not only have in all probability a common origin, but also 

 that they are mutually more closely related. The groups which 

 are thus brought together under the phylum of Ameria show 

 the following common traits if compared with the next higher 

 phylum: (1) Not a single class of all these fifteen classes has 

 a perigastrocoele, thus a coelom s.str. (2) We cannot find 

 anywhere in this phylum a genuine segmentation and a 

 formation of body regions which would appear in combina- 

 tion with this segmentation. (3) None of these classes has 

 ever developed any extremities, and they have therefore not 

 developed any articulated extremities. (4) Their protonephridia 

 had developed into metanephridia only by Molluscs as a conse- 

 quence of the union wath the increased gonocoel. The first three 

 characteristics represent negative values only, yet they can 

 also be expressed in a positive way, i.e. (1) these classes 

 possess only the coeloms s.L\ (2) their body is always homo- 

 geneous (the amerous state); and (3) they can move in various 

 ways (by means of cilia or of body muscles), yet never by 

 means of genuine extremities. 



As could be expected there appear occasional charac- 

 teristics, at least as Anlagen^ scattered among the classes of 

 Ameria which had played a very important role in the evolu- 

 tion of the Polymeria, the next higher animal type. As an 

 example, we can mention the polymerization of individual 

 organs (gonads, nephridia, etc.) or of whole parts of the body 

 (Cestoda), an increased growth into length combined with 

 a curving movement, etc. 



