CHAPTER XVIII. 



CLASS CEPHALOCHORDATA. AMPHIOXUS. 

 (Syn. Leptocardii, Acrania.) 



The lancelet (Amphioxus) is found near the coast in most 

 warm and temperate seas. It is pointed at both ends as its 

 names suggest, it rarely measures two inches in length. It 

 has a faint flesh colour, and is translucent. 



The lancelets are fond of lying in the sand in water about 

 two fathoms deep, all covered except the fringed aperture of 

 the mouth, through which diatoms and other small organisms 

 are sucked in. In the evening especially, they sometimes 

 start up and swim about, bending the body in wriggling 

 motion. The young swim about more than the adults. 



Characteristics. — There is no doubt that Amphioxus is far 

 removed from the type of Fishes. It has no limbs, no 

 skull, no jaws, no well-defined brain, no sympathetic nervous 

 system, no ear, no heart, no spleen, no kidneys, no repro- 

 ductive ducts. The notochord persists unsegmented, 

 surrounded by a continuous sheath. A median fin extends 

 along^he back,, around the tail, and for some distance along 

 the ventral surface. But in its dorsal nervous system, 

 notochord, and gill-slits, Amphioxus exhibits the essential 

 Vertebrate characters. There is but one genus with about 

 half a dozen species, of which A. lanceolatus is best known. 



Description of Amphioxus lanceolatus. 



Form. — The body is thin and sharp at each end. The 

 muscles are disposed in sixty-two segments or myotomes. 



