prot( 



l.rnus 



$et.sncs 



prof. 



prvt 



pig.ar 



an 



. h 



^L -fr.loJ. 



FIG. 356. Nereis. Later stages in the development. A, stage at which the prototroch and the 

 apical tuft of cilia first become distinct. B, somewhat later stage, in which the stomodajal 

 invagination is being formed, and the rudiments of the mesoderm bands are distinct ; C, late 

 trochophore stage in which there are rudiments of the setigerous sacs ; Z>, somewhat later 

 stage, in which the parapodia have begun to become prominent and the provisional setsv. 

 project freely ; E, larva with three segments, an. anus ; up. fit. apical cilia ; ap. pi. apical 

 plate ; eye, eye ; j'r. bod. frontal bodies ; int. intestine ; 1. mug. longitudinal muscle ; mes. 

 mesoderm ; mo. mouth ; neur. pi. neural plate ; para, parapodia ; pig. ar. pigmented area ; 

 prof, prototroch ; sens. h. sensory hairs ; set. sacs, setigerous sacs : som. second somatoblast and 

 group of cells formed from it ; st. stomodfeum ; tent, peristomial tentacles. (After E. B. 

 Wilson.) 



453 



