484 ESSENTIALS OF ZOOLOGY 



The larvae of all echinoderms possess bilateral symmetry. These 



larvae all have definitely arranged bands of cilia over the body. 



Embryologists have pointed out the fact that these larvae are not 



directly related to the trochophore. They show more resemblance to 



the Tornaria larva of Balanoglossns than to any of the nonchordate 



forms. 



Ancestry of the Vertebrates 



The establishment of relationships between the chordate and non- 

 chordate animals has been one of the perplexing problems in the 

 study of phylogeny. Different students of this problem have in- 

 vestigated the possible relationships of such nonchordate groups as 

 the flatworms, nemertine worms, annelids, arachnids, insects, and 

 echinoderms. Their investigations have resulted in the formulation 

 of a number of theories putting forth the various ones of the above- 

 mentioned groups as the progenitors of, or claiming common an- 

 cestry with, chordates. 



Some authors insist that the chordates have arisen from some 

 segmented form ; others conclude from their evidence that this is 

 not necessarily true or essential. All of the theories establish their 

 relationships to the vertebrates through the protochordates, which 

 are represented by Amphioxus, the tunicates, and Balanoglossns. 

 Each of these has been considered as ancestral stock, which has 

 contributed to the origin of vertebrates. It is usually conceded by 

 authorities in this field of study that Amphioxus is a modified an- 

 cestor of the vertebrates, due to the clear-cut and well-defined con- 

 dition of the distinctive characteristics of the chordates and the 

 presence of a midventral endostyle. Next in the line of thought 

 would come the possibilities of ancestors of Amphioxus. The tuni- 

 cates have been given this distinction by some. The adult has lost 

 most of its typical primitive characteristics, but the larva possesses 

 the distinctive characteristics of chordates and also the ventrally 

 located endostyle, very similar to these structures in adult Am- 

 phioxus. It has been suggested that probably the adult tunicate 

 once existed as an animal similar to its larva of today, and that 

 its forebear was not only the ancestor of modern tunicates but also 

 the form from which the Amphioxus group has descended. 



Balanoglossns, which is usually considered the most primitive of 

 chordates, is regarded as a possible ancestor of, or as possessing 

 common ancestral stock with, tunicates and Amphioxus. As will 



