PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 867 



Eacli 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. 



Hemichordata, which is usually considered the most primitive of 

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

 common ancestral stock with, tunicates and Amphioxus. As will 

 be remembered from the previous study of Hemichordata, its repre- 

 sentatives possess gill slits, a support in the base of the proboscis 

 which may be homologous to the notochord, and four longitudinal 

 nerve cords of which the dorsal is the most highly developed. The 

 above protoehordate relations are rather generally conceded, but there 

 is much less agreement concerning their origin, and several theories 

 have arisen of which the following are important. 



Annelid Theory.— The segmental condition of this group, the re- 

 lationship of the digestive sj^-stem to other circulatory and nervous 

 systems, and presence of the coelom with related nephridia, present 

 a close comparison to what is found in the embryonic development 

 of the vertebrates. It has been suggested by some scholars that by 

 inverting the body of the nonchordate annelid, the fundamental 

 systems are brought to resemble their relative locations in verte- 

 brates. A fibrous cord has been found in some groups of the an- 

 nelids, and this structure is held to take the place of a notochord in 

 function and position. These fibers are found just dorsal to the 

 chain of ganglia in the annelids. 



Arachnid Theory. — Such forms as scorpions, Limulus, and other 

 arachnids have been favorably compared to vertebrates. By com- 

 parison of these arachnids with the extinct, fishlike ostraeoderms, 



