THE NEW GENEALOGICAL TREE 401 



to the Ectoprocta as has been done by various zoologists 

 (e.g. by Marcus and by Hyman) as if the "Bryozoa" evolved 

 from the Kamptozoa parallell to a development of an undivided 

 coelom cavity. This is clearly a case of a pure parallelism only, 

 which took place as a consequence of a similar one-sided 

 way of life at two entirely different levels of the general 

 organisation. The polymerous Annelida can be considered 

 as the direct ancestors of the Ectoprocta while at the same 

 time they are genuine Oligomeria which show the extreme 

 level reached in the retrogression of the perigastrocoele. 



A clear trend towards a reduction of the segmented coelom 

 system can also be observed in the evolution of the coelom 

 conditions in the second large subdivision of the Polymeria, 

 in the Arthropoda. These adopted movement by means of 

 articulated extremities while at the same time the length of 

 their body was decreased. As a final result of this reduction 

 we find one undivided body cavity, the myxocoele, which 

 is clearly a secondary element and which is also united with 

 the primary body cavity. Something similar had also taken 

 place in the Hirudinea, among the Annelida, where again it 

 appears together with a change in the type of movement. 



All the facts that can be found in the field of the compara- 

 tive morphology of the adult Polymeria therefore support the 

 interpretation that the perigastrocoele evolved first in the 

 form of numerous pairs of small sacks, and thus as a segmented 

 form. It was only due to a retrogression of this system (oligo- 

 merization) that the number of segments were decreased, or 

 even that segmentation had completely disappeared. It is 

 therefore quite improbable that the polymerization of the 

 perigastrocoele had taken place step by step, thus by way 

 of an oligomeric state (zoologists have usually thought here 

 of a trimery). 



At the beginning of our present study wx have already 

 discussed the problem of the relation betw^een the ontogeny 

 and the phylogeny. It is obvious that we must proceed very 

 carefully when we consult the ontogenetic morphogenies as 



