150 



INVERTEBRA TE ZOOLOGY 



from those of the front half of the same worm. Thus an anterior 

 atoke and a posterior epitoke are recognizable in the body of the 

 mature Nereis, though the two regions never become separated 

 as in the palola worm. The most conspicuous of the changes in 

 the epitoke region concern the parapodia. These increase in 

 size while the original setae are shoved out and are replaced by 

 new ones of entirely different shape and arrangement. The 



lobes of the parapodia develop 

 new outgrowths which render 

 them more effective swim- 

 ming organs. Even though 

 the parapodia of the immature 

 Nereis seem to be admirably 

 fitted for swimming, the 

 worm rarely becomes pelagic 

 except at the onset of sexual 

 maturity. 



Development. — Practically 

 all polychaetes are marine 

 and in development pass 

 through the trochophore 

 stage. In organization, the 

 polychaetes represent a more 

 primitive condition than the 



Fig. 76. — The development of Poly- oligochaetes. Though the 



gordius. A, young trochophore; fi.elonga- ^y ip-^pViaptPK flrp frpoupntlv 



tion of posterior cone; c, D, stages in oDiigocnactes are irequentiy 



transformation of trochophore; £, anterior simpler in strUCturC, their 

 extremity of young Polygordius following „;^„14„;^^ :„ ^.y^p r-pmilt of 

 metamorphosis. {After Fraipont). Simplicity IS tne reSUlI OI 



degeneracy, for it seems prob- 

 able that in becoming adapted to life in fresh water or to the 

 terrestrial existence they have departed from the generalized 

 structure and mode of development which characterize the more 

 primitive members of the class. 



The trochophore (Fig. 76, A) is typically pear-shaped or in 

 the form of two cones joined by their bases, one dorsal and the 

 other ventral in position. Surface ciliation of this larva may 

 follow any one of a number of patterns. A tuft of cilia at the 

 apical plate and a preoral circle near the equator of the larva 

 are the most constant, but other bands may also appear. The 

 digestive system consists of a mesenteron which communicates 

 with the mouth opening through an esophagus and with a 



