202 COMPARATIVE ANATOMY 



characters of vertebrate organisation, differs from all other 

 members of the phylum in several important characters, chief 

 among which is the absence of a specialisation of the anterior 

 end of the body to form a definite head. In the absence of 

 the head, all structures associated with a head are also absent, 

 paired eyes, paired auditory organs, a definite brain, and jaws. 

 Because of this, Amphioxus is sometimes placed apart from all 

 other vertebrates in a section Acrania. But this method of 

 classification is not without inconveniences. As a matter of 

 fact, Amphioxus cannot properly be called a vertebrate, for it 

 has no vertebral column composed of separate vertebrae, but 

 only the precursor of a vertebral column — namely, a noto- 

 chord. But a notochord is a possession common to the true 

 vertebrates, to Amphioxus, to the group of animals known as 

 " sea-squirts," Ascidia, or Tunicata, and to some other groups, 

 and we may therefore group all these in a phylum Chordata. 

 In this phylum Amphioxus is characterised by the forward 

 extension of the notochord to the anterior end of the body, 

 and therefore it and the genera immediately related to it, form 

 a class Cephalochorda. The Ascidians having a notochord 

 only in the tail, form a class Urochorda : certain worm-like 

 animals with a very imperfect notochord are Hemichorda, and 

 the true vertebrates possessed of a definite head are Craniata. 



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