CHAPTER XIX. 



PHYLUM CHORDATA. 



SUB-PHYLUM CEPHALOCHORDA. 



(Synonyms ACRANIA, LEPTOCARDII, PHARYNGOBRANCHII.) 



THIS small sub-phylum includes about sixteen species, 

 popularly known as lancelets. The type represents 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 somew r hat degenerate pioneer. 



GENERAL CHARACTERS. 



There is a dorsal tubular nerve-cord, but 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, and open into an 

 a trial or peribranchial cavity. The body wall is built up of 

 over fifty myotomes. From Fishes, the lancelets are widely 

 removed by the absence of limbs, skull, jaws, differenti- 

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

 heart, spleen, and genital ducts. There are numerous 

 separate nephridia. The gonads are numerous and arranged 

 segmentally. The larval form is strangely asymmetrical and 

 the larval period is prolonged. The species have a wide 

 distribution, like many old-fashioned animals. They occur 

 near the coasts in warm and temperate seas, are sluggish in 

 hain't, and feed on microscopic organisms or organic particles. 



