118 NEMERTINEA [CH. VI 



which are derivatives of the ectoderm cells, but outside all the 

 other muscles : no stylet in the proboscis. 

 Ex. Linens, Cerebratulus. 



Class IV. METANEMERTINI. 



No nervous sheath but definite peripheral nerves : the 

 lateral nerve-cords lie within all the muscle layers : the 

 proboscis is armed with a stylet. 



Ex. Tetrastemma, Geonemertes, Malacobdella. 



Classes I III are often united as the Class ANOPLA, charac- 

 terised by the absence of a stylet in the proboscis. Class IV is 

 sometimes termed ENOPLA (i.e. armed) to contrast it with the 

 remaining Nernertines or Anopla. 



