CHAPTER XIX 
PHYLUM CHORDATA 
SUB-PHYLUM CEPHALOCHORDA 
(Synonyms—ACRANIA, LEPTOCARDI, PHARYNGOBRANCHII) 
Tuis 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 somewhat degenerate pioneer. 
GENERAL CHARACTERS 
There is a dorsal tubular nerve-cord, but no well-definea 
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. 
Ln the adult the gill-slits are very numerous, and open into an 
atrial or pertbranchial 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, differentiated 
brain, sympathetic nervous system, eye, ear, definite heart, 
spleen, and genital ducts. There are numerous separate 
nephridia. The gonads are numerous and arranged seg- 
mentally. 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 
habit, and feed on microscopic organisms or organic particles, 
