542 Causes and Course of Organic Evolution 



Its derivation as a group from an advanced nemertean and 

 protochordate ancestry is suggested by the well-developed 

 gill system with gill-pores and branchial sacs, by the blood- 

 vascular system and other details that cannot here be dis- 

 cussed. So, as the illustrations suggest, it can best be viewed 

 as a derivative from a decidedly specialized marine nemertean 

 and protochordate offshoot of the originally fresh-water nemer- 

 teans, that shows structural tendencies of an evolving and a 

 devolving kind, which together now typify the class. 



The members of the Urochordata that we know as free- 

 swimming ascidians represent a greatly more advanced type 

 of organization than the last, and also are derivatives from a 

 more direct nemertean-chordate stock than are the last. Thus 

 the embryological stages in formation of the notochord, of 

 the neural canal, of the body cavity, and other organs are 

 exactly and typically of a "chordate" character. But the 

 group as such furnishes no key as to the causes for origin of 

 the notochord, for massing of the nervous centers and main 

 nerve-trunk on the anterior dorsal region, or for the origin 

 and position of the branchial sacs. All of these become clear 

 when we refer them back to a nemertean ancestry, and to the 

 relation there of proboscis-sheath and proboscis with the future 

 notochord. 



But the free-swinmiing ascidians are exceptionally inter- 

 esting as being a group that fairly perfectly connects the nemer- 

 teans with Amphioxus and the cyclostomes. As to their 

 derivation, the view seems most consistent with all the details 

 of their structure, which would link them as derivatives from 

 a marine protochordate ancestry that itself originated from 

 a fresh- water type intermediate between the fresh- water nemer- 

 teans and a primitive ancestor of the cyclostomes. The de- 

 generation that has taken place through fixation or sluggish 

 response to environal stimuli, even in colonial groups, has 

 caused the entire class to become a striking biological cul-de- 

 sac. Secretion of defensive materials round the body has pre- 

 vented extinction, and even contributed to increase and geo- 

 graphical distribution. The position of the class, as now esti- 

 mated, is set forth in the chart. 



