CHAPTER XIX. 



CEPHALOCHORDA. 

 (Synonyms — Acrania, Leptocardit, Pharyngobranchii.) 



This small class includes Amphioxus (Epigonichthys) and 

 Asymmetron, popularly known as lancelets. The type re- 

 presents an offshoot from the primitive Vertebrate stock, 

 lost, it is to be feared, for ever ; but while some authorities 

 regard it as a pioneer-type and as a far-off prophecy of a fish, 

 others hold it to be degenerate — a " weed in the Vertebrate 

 garden." It is possible that both views are right, and that 

 the lancelet is a somewhat degenerate pioneer. 



General Characters of Cephai.ochorda. 



The nervous system has no well-defined brain region. The 

 notochord is persistent and unsegmented ; it is surrounded by 

 a continuous sheath, and projects in a unique manner in front 

 of the anterior end of the nerve-cord. In the adult the gill- 

 slits are very numerous. From Fishes, the lancelets are widely 

 removed by the absence of limbs, skull, jaws, differentiated 

 brain, sympathetic nervous system, eye, ear, definite heart, 

 spleen, and genital ducts. The species have .a wide distribu- 

 tion, like many old-fashioned animals. They occur near the 

 coasts in warm and temperate seas, are sluggish in habit, and 

 feed on microscopic organisms or organic particles. 



Amphioxus lanceolatus, the best-known species. 



Mode of life. — The lancelets are fond of lying in the 

 sand in water about two fathoms deep, with only the fringed 

 aperture of the mouth projecting. They feed on diatoms 



