432 THE EVOLUTION OF THE METAZOA 



semi-sessile animals in canals which they burrow in the sea 

 floor. At the same time, however, they have also preserved 

 some traces of a free mobiUty; animals have been observed 

 that swam over short distances by means of sinuous move- 

 ments, a fact which I wish particularly to call attention to. 



The standpoint which can be defended here is that the con- 

 ditions that can be observed in the Sipunculida represent the 

 final stage of an evolution which led from the freely moving 

 Annelida by way of the tubicolous and burrowing Annelida 

 to the Echiuroidea and finally to the Sipunculida. The simple 

 conditions that can be observed in the Sipunculida are therefore 

 not a primary phenomenon. In the Echiuroidea we can still find 

 several peculiarities of the Annelida clearly developed, thus 

 above all the polymery and the neuromery which are tempora- 

 rily recapitulated in their ontogeny, as well as the metanephri- 

 dia and the remains of a parapodial apparatus, etc., while at the 

 same time all these peculiarities of the Annelida have been lost 

 in the Sipunculida. There is clearly a sharply-cut limit between 

 the Echiuroidea and the Sipunculida which becomes even 

 more clear if we liberate ourselves definitively from the notion 

 of "worms" ("Vermes") as a systematic category which has 

 now completely lost its significance. 



The Sipunculida which have remained to a lesser degree 

 freely moving animals had evolved as such from a simplified 

 structure into a specialized direction; during this evolution 

 they had developed many new properties, and this is the 

 reason why they have been considered, correctly, as an in- 

 dependent higher systematic category (a class, frequently even 

 as a phylum which seems to me an overvaluation). They also 

 show more and more clearly, besides the prosopygy which is 

 a consequence of their burrowing and sessile way of life, 

 another peculiarity w^hich is characteristic of all the Oligo- 

 meria ; the division of their bodies into two regions (the pro- 

 soma and the metasoma). The anterior region which is fre- 

 quently divided into two parts shows the strongest develop- 

 ment in the most primitive groups of the Oligomeria, and 



