464 



ZOOLOGY 



SECT 



groove. There is a vascular system with dorsal and ventral longitudinal vessels. 

 In each segment is a pair of simple nephridia. In Protodrilus there are two 



Br 



FIG. 368. Trochosphere of Polygordius neapolitanus ^4, lateral view of entire larva ; 

 B, diagrammatic vertical section ; C, transverse section through the plane ab in B ; An. anus ; 

 An. ci. anal cilia ; Bl. blastoccele ; Br, apical plate ; Ent. enteron ; Msd. mesoderm ; Msd. 

 bd. mesodermal bands ; Nph, head-kidney ; Oc, eye-spot ; Pr. or. ci. cilia of prototroch ; Prc. 

 dm. proctodseum ; Pt. or. ci. post-oral cilia ; St. dm. stomodseum ; V. Nr. C'd. ventral nerve cord. 

 (From Parkers Biology, partly after Fraipont.) 



Mid (ft 



V.WtCd 

 Pr.ftn.ci~ 

 An.ci 



FIG. 369. Later stage in the development of Polygordius neapolitanus, in which the 

 posterior part of the trochosphere has become elongated and segmented ; A, entire larva ; 

 B, vertical section ; c, transverse section along the plane ah in B ; D* J)3. three stages in the 

 development of the somatic mesoderm ; CW. crelome ; cce.l. Epthm. coalomic epithelium ; Der. 

 Epthm.d&fic epithelium; M.,,l. muscle plate; M.l. (.>, ,,.\ somatic mesoderm; M*<i. (v y) /\ 

 splnnchnic mesoderm ; 0. eye ; t. tentacle. Other letters as hi preceding figure. (From 

 Parker's Bioloyii, partly after Fraipont.) 



ventral nerve-cords, connected together by transverse commissures : in Polygor- 

 dius the cord (Fig. 367, V. Nv. cd) is single ; in neither genus is there any trace 



