260 INVERTEBRATE ZOOLOGY 



of the more important ones have been chosen for discussion. 

 The coelenterates, the nemertines, the annehds, and various 

 arthropods have been championed as the possible origin of the 

 vertebrate series. 



Annelid Theory. — The metameric condition of annelids, the 

 relation of their nephridia to the coelom, and the fundamental 

 relationships of vascular and nervous systems to the digestive 

 tract closely resemble the conditions found in the lower verte- 

 brates and in vertebrate embryos. Semper and other investiga- 

 tors have shown that by inverting the position of the body of 

 an annelid the fundamental systems are brought into almost 

 complete harmony with their arrangement in the vertebrates. 

 Such a shift in the orientation of the body is not at all uncommon 

 in various animal groups. Many of the crustaceans and molluscs 

 move with their ventral surface in a dorsal position. Even the 

 notochord, which is distinctive for the Chordata, has a counter- 

 part in the bundles of supporting fibers which accompany the 

 annelidan nerve chain. Recent publications of Delsman have 

 awakened a new interest in the annelid theory of vertebrate 

 origin. 



Nemertine Theory. — Hubrecht has maintained that the nemer- 

 tines stand in the direct line of ancestry of the vertebrates. 

 One of the chief arguments in favor of this theory is the possible 

 homology between the proboscis sheath of the nemertine and 

 the notochord of the chordate. The lateral nerve cords of 

 the nemertine could assume a dorsal position as in the chordates 

 without a complete change in the orientation of the body such 

 as is necessitated in the annelid theory. 



Arachnid Theory. — Patten has seen in the arachnids, especially 

 in forms like the scorpions and Limulus, many points of structure 

 in direct harmony with vertebrate organization. Through 

 comparisons of these arachnids with fishlike Ostracoderms he 

 has built up an elaborate theory showing a possible origin of 

 the vertebrates from arachnidan ancestry. 



Conclusion. — Each of the numerous theories, of which the 

 foregoing are characteristic examples, is based upon a group 

 of facts derived from studies in comparative anatomy and embry- 

 ology. Yet no one theory depicts a satisfying genealogy of 

 the vertebrate group. At most, the various hypotheses furnish 

 ground for a belief in kinship between the vertebrate and the 

 invertebrate. Kinship or common origin would explain most 



