144 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



SUBBRANCH 3. Vertebrata 



DIVISION A. Acrania 

 Vertebrates without a skull. 



CLASS. Pharyngobranchii 



The Acrania are represented by the singular animal 

 Amphioxus or lancelet. It is about two inches long, 

 semitransparent, without skull, limbs, brain, heart, or red 

 corpuscles. It has for a skeleton a notochord only. It 

 breathes by very numerous gill arches, without fringes, 



FIG. 117. Lancelet (Amphioxus). Notochord, c ; spinal cord, sc ; oral tentacles,/; 

 gills, g ; ovary, ov ; liver, / ; anus, a ; pore of branchial chamber, p ; muscle plates, 

 m ; tail fin,/. Natural size. 



and the water is drawn in by cilia, which line the gill 

 slits. The embryo develops into a gastrula closely 

 resembling that of the invertebrates. The animal lives 

 in the sandy bottom of shallow parts of the ocean, and 

 has been found in the Mediterranean Sea, in the Indian 

 Ocean, and on the coasts of North America and South 

 America. 



DIVISION B. Craniata 



Vertebrates with a distinct skull. 



* 



CLASS I. Cyclostomata 



The lampreys 

 and hagfish have 

 a persistent noto- 

 chord, a cartilagi- 

 nous skull, no lower 

 jaw, a round, suc- 



Fic. 118. - Lamprey (Petromyzon marinus) . Atlantic. tOlial niOUth, homy 



