PHYLOGENY 259 



ancestral form have considered it as a modified trochophore 

 not fundamentally different in structure from some of our present- 

 day rotifers. The swimming appendages of rotifers like Pedalion 

 are thought by some to represent incipient arthropod appendages. 



Ancestry of the Vertebrates. — Some of the most widespread 

 interest in the blood relationships of animals centers around the 

 question of the origin of the Chordata. Even though the verte- 

 brates seem to have made their appearance at a time when a 

 fairly complete picture of the fauna is observable through the 

 fossil remains, there is little light thrown upon the origin of 

 the vertebrates through the study of paleontology. Fossils 

 of some of the heavily armored fishes, the Ostracoderms, closely 

 resemble the fossil arachnids known as the Merostomata and our 

 modern Limulus which some one has called a "living fossil." 

 The study of comparative anatomy, however, seems to indicate 

 that the vertebrates must have had a simpler beginning than this 

 would indicate. 



The three subphyla of the Prochordata, represented by 

 Amphioxus (or Branchiostoma), Balanoglossus, and the tuni- 

 cates, have characters which permit them to be classified with 

 the Chordata yet in most instances display a low type of general 

 organization which seems to relate them to the non-chordate 

 forms. Each of these groups has been considered as a possible 

 ancestral stock from which the true vertebrates have had their 

 origin. Since these forms lie outside the scope of this textbook, 

 their relationships to the problem of vertebrate phylogeny will 

 not be considered in detail, but attention will be directed to some 

 of the invertebrate groups through which the ancestry of the 

 vertebrates has been derived by various investigators. 



As has been pointed out in an earlier discussion, the Metazoa 

 in general, including the Vertebrata, develop from a single 

 cell, the zygote. Thus a possible ancestral history of the verte- 

 brates leads back to the single-celled organisms. This rela- 

 tionship is so distant, however, that many attempts have been 

 made to find satisfactory evidences of vertebrate geneology 

 through the higher groups of the invertebrates. Early metamer- 

 ism of the vertebrate embryos indicates that some segmented 

 organism gave rise to the vertebrate line, but further than this 

 the evidences are capable of broadly divergent interpretations. 

 In consequence, numerous theories have been advanced. Space 

 does not permit a detailed account of all of these but some 



