CHAPTER II 

 THE PHYLOGENESIS OF VERTEBRATES* 



" The Epicureans, according to whom animals had no 

 creation, doe suppose that by mutation of one into 

 another, they were first made; for they are the sub- 

 stantial part of the world; like as Anaxagoras and 

 Euripides affirme in these tearmes : nothing dieth, but 

 in changing as they doe one for another they show 

 sundry formes." 



PLUTARCH'S Morals; transl. by Philemon Holland, 



1603, p. 846. 



ALTHOUGH no great subdivision of animals, with the pos- 

 sible exception of the echinoderms (star-fish, sea-urchins, 

 etc.), possesses a more isolated position than do the verte- 

 brates, this latter group is connected in an obscure way with 

 the invertebrate world through a series of animal forms of 

 uncertain position themselves and usually grouped together 

 under the name of Prevertebrata or Protochordata. These 

 comprise a worm-like form, Balanoglossus, that burrows in 

 the mud along the sea-coasts, the sac-like tunicates, and the 

 small and slender Amphioxus. Formerly classed at great 

 distances from one another among molluscs, worms and even 

 plants (e. g. f sessile tunicates), they are now united, owing to 

 the common possession of pharyngeal gill-slits, a dorsal 

 nervous system, and an internal skeletal rod, the notochord, 

 although in some cases these two latter characteristics are 

 transitory structures that appear only during the early steps 

 of development. 



The highest of these animals, and consequently the one 

 nearest the true vertebrates, is Amphioxus, a small marine 

 creature something like a headless fish, which is found in the 



* For a detailed classification of vertebrates, to accompany this chap- 

 ter, the reader is referred to the Appendix 



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