CHAPTER XIX. 



CEPHALOCHORDA, AMPHIOXUS. 

 (Synonyms, Acrania, Leptocardii, Pharyxgobranchii.) 



There is but one well-defined genus in this class, — the 

 lancelet {Amphioxus). Yet it deserves a class for itself, so 

 distinct is it from other Chordata. It may be regarded as 

 an offshoot from the primitive Vertebrate stock, lost, it is to 

 be feared, for ever, as a far-off prophecy of a fish, or, per- 

 haps, as a degenerate type, " a weed in the Vertebrate 

 garden." 



General Characters of Cephalochorda. — A class 

 of simple Chordate animals, represented by one distinct type — 

 Amphioxus. The nervous system has no well-defined brain 

 region. The notochord is persistent and unsegmented : it is 

 surroimded 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, Amphi- 

 oxus is ividely removed by the absence of lind'S, skull, jaws, 

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

 heart, spleen, and genital ducts. The species have a wide 

 marine distribution near the coasts in warm and temperate 

 seas, are sluggish in habit, and feed on microscopic organisms 

 or organic particles. 



Description of Amphioxus lanceolatns, 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 



