PHYLOGENY 



PPENING OF PERIBRANCH.CAV. 

 /ANUS 



NERVE CORD /NOTOCHORD 



PAPILLAE 



ENOOSTYLE 



GILL SLfTS'^ 



I HEART 



A. LARVAL UROCHORDATE 



^DORS*L NERVE 



NOTOCHORD 



GILL SLITS' 



1 LIVER 



ANUS 

 POST OPENING OF 

 PERIBRANCHIAL CAVITY 



B. CEPHALDCHORDATE 



NOTOCHORD 



MOUTH/ THYROID' I^^ill SUTS ^HEART ^LIVER 



INTESTINE 



C. VERTEBRATE (CYCLDSTOME) 



Figure 53. Primitive Chordates. (From Neal and Rand, "Comparative Anatomy, 

 The Blakiston Co., 1939.) 



that Amphioxus is quite specialized in some characters, such as the in- 

 clusion of the gills within an atrium, the fact that the notochord runs the 

 entire length of the body, and the inexplicable fact that the kidneys of 

 Amphioxus have more in common with those of annelids than they do 

 with those of vertebrates. These primitive chordates (Figure 53) must 

 have branched off from the main chordate stock very early, and their 

 exact relationship to the vertebrates is no more clear today than it was 

 when the question was first raised. The fossil record is of no help, for 

 none of the prochordates are known as fossils at all (unless Jamoytius 

 qualifies ) . 



The vertebrates themselves are divided into eight classes, of which one 

 is extinct. Four of these are entirely aquatic, the classes Agnatha, Placo- 

 dermi, Chondrichthyes, and Ostcichthyes. They may be grouped together 

 as the superclass Pisces, corresponding to the common term "fish," yet 

 they differ from one another more fundamentally than do the remaining 

 four classes of land animals, which everybody recognizes as distinct. 

 These are the classes Amphibia, Reptilia, Aves, and Mammalia, and they 

 comprise the superclass Tetrapoda. 



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