420 



THE EVOLUTION OF THE METAZOA 



burrow, eating sediments that are rich in organic substances, 

 or they try to find protection in clifF fissures, empty shells 

 of snails, etc. The parapodia and the sinuous type of locomo- 

 tion became superfluous to these animals; this again led to a 

 considerable reduction of their organization which had even 

 reached the internal polymerization. The consequence of this 

 development had been the loss of the dissepiments and of the 



Fig. 57, Schemes illustrating the transition from Polymeria to 

 Oligomeria: A, echiurid; B, sipunculid; C, phoronid; D, ecto- 



prokt. 



chaetae. Even among the recent species we can observe all the 

 stages of this reduction which finally led to the emergence of 

 a new animal type. These stages are well represented by the 

 following forms: Arenicola-Sternaspis (with the disintegration 

 of the dissepiments)-jE^>&//^r//j' (the segmented perigastrocoele 

 can be found in their embryos ov\f)-Sipunculus-Phoronis (Fig. 

 57). We will later return to discuss the Sipunculoidea in more 

 detail. The elongated worm-like form had been preserved by 

 all these animals in spite of the loss of an external and of an 

 internal segmentation. We frequently hear in this connection 



