166 The Leptocardii, or Lancelets 
into a hollow tube.” Besides this trait two others are common 
to all of them, the presence of the gill-slits and that of the noto- 
chord. The gill-slits may have served primarily to relieve the 
stomach of water, as in the lowest forms they enter directly into 
the body-cavity. The primitive function of the notochord is 
still far from clear, but its ultimate use of its structures in 
affording protection and in furnishing a fulcrum for the muscles 
and limbs is of the greatest importance in the processes of life, 
Fic. 116.—Gill-basket of Lamprey. 
