THE NEW GENEALOGICAL TREE 449 



ancient fossil remains belonging to the Chordoma, has been 

 found in the sediments which had been deposited by fresh 

 water. The second necessary presupposition is that during 

 the phase of transition the ancestors of the Enteropneusta 

 which had evolved into the Chordoma had begun to move 

 more intensively as freely moving animals. This change was 

 probably connected with an adaptation of these animals to 

 a life in firmer and coarse-grained sand sediments after they 

 had left the soft mud and the fine sand. The incUnation to 

 swim which had never been completely lost, now became 

 reactivated. Simultaneously, we can observe a rich secondary 

 segmentation in the dorsal side of their oblong bodies which 

 had become strengthened due to these changes; this segmen- 

 tation had taken place above all in the body muscles and in 

 the nerve trunk which had already become extended along 

 the dorsomedian line. Other organs placed more or less 

 dorsally, as the previously uniform perigastrocoele, the gonads, 

 and the nephridia afterwards also participated in this segmen- 

 tation. 



The consequence of the special type of swimming which had 

 been developed more and more progressively by the freely 

 moving Chordoma— wdth the exception of the Tunicata 

 which had secondarily returned to a sessile way of life — had 

 been a special new formation which had developed at the 

 end of their bodies. Here a new zone of growth emerged whose 

 consequence was a new body region, that of the tail. The 

 more active type of movement led to a progressive develop- 

 ment of the two systems which had been inherited from the 

 enteropneustan ancestors, the breathing system and the system 

 of blood circulation. The trend to develop folds in the region 

 of the breathing organs, which can also be observed in the 

 Enteropneusta, had been continued in the lower Chordoma 

 (the atrial cavity for the protection of the gill apparatus); 

 yet this cavity later became more and more retrograded in 

 fishes, etc., because of the completely freely swimming way 

 of life which these animals had adopted. 



