ANCESTORS AND INTERRELATIONS OF VERTEBRATES 693 



link which may connect this lowest of the chordates with the 

 invertebrates and thus complete our hypothetical line of 

 vertebrate descent. The egg of, the enteropneuston Balano- 

 glossus develops into a small larva called Tornaria (Fig. 334), 

 whicli floats in the sea, is transparent, has a bilateral sym- 

 metry, and is provided with bands of cilia for locomotion. 

 This larva corresponds in habitat and structure almost exactly 

 to the larvae of the starfish and other echinoderms. This 

 similarity leads to the conclusion that a form resembling these 

 larvae was the very remote progenitor of both the echinoderms 

 and the chordates, and that " The lineal descendants of this 

 hypothetical ancestor chose two paths, the one leading to the 

 ECHINODERMATA, the other to Balanoglossus, the TUNICATA, 

 Amphioxus, and eventually the VERTEBRATA." 



" The question of the descent of the CHORD ATA is not solved 

 by accepting their relationship to the ENTEROPNEUSTA, since 

 this latter group holds an uncommonly isolated position. Only 

 from the structure of the Balanoglossus larva can there be con- 

 cluded a distant connection with the echinoderms. We must 

 resign ourselves to the thought that at the present time we are 

 not in a condition to assert from what ancestral form the CHOR- 

 DATA, and with them Balanoglossus, are to be derived. The 

 origin of the vertebrates is lost in the obscurity of forms un- 

 known to us." (Wilder.) 



2. THE PHYLOGENESIS OF VERTEBRATES 1 



Anatomical and paleontological investigations are continually 

 changing our ideas regarding the interrelations of the verte- 

 brates, and we can indicate only provisionally the possible line 

 of descent of the vertebrates and the relations of one group to 

 another. Reference to Figure 551 will make the following 

 paragraphs clear. 



The lowest vertebrates, i.e. the forms most nearly related to 



1 For a more detailed account of this subject, see Wilder's History of the Hu- 

 man Body, Chapter II. 



