The Leptocardii, or Lancelets 



485 



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. 



Fig. 289a, — Gill-basket of Lamprey. 



