PHYLUM ANNULATA 



505 



system and in the presence of ciliated pits ; but it has a row of comb- 

 like sjttv on each segment, and it has only a single pair of nephridia, which are 



Br 



B 



Prc. t?m. 



An ci 



-An.ci 



Fi<;. 402. Trochophore of Polygordius neapolitanus. A, Literal view of entire larva ; 

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

 An. ci. anal cilia; Bl '. blastocdele ; Br. apical plate; Ent. enteron ; Msd. mesoderm ; Msd. 

 Id. mesodennal bands ; Nplt. head-kidney ; Or. eye spot ; Pr. or. ci. cilia of prototroch ; Prc. 

 il,,t. proctodsBum ;Pt. or. ci. post-oral cilia : St. </m. stomodaeuni ; V. Nv.Cd. ventral nerve-cord. 

 (From Parker's Biology, partly after Fraipont.) 



An.ci 



FIG. 403. Later stage in the development of Polygordius neapolitanus, in which the 

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

 B, vertical section ; C, transverse section along the plane nb in B ; D l D 3 , three stages in the 

 development of the somatic mesoderm : Cut. ccelome : cnl. Eptlini. cceloinic epithelium ; Der. 

 Eptltni. deric epithelium; M./>1. muscle-plate: Mx<i. (xnt.), somatic mesoderm; Mm!. (</'/), 

 splanchnic mesoderm; 0. eye; t. tentacle. Other letters as in preceding figure. (From 

 Parker's Eivlogy, partly after Fraipont.) 



situated in the peristomium. There is only a single (longitudinal) layer of 

 muscles in the botty-wall ; the reproductive apparatus is not known. 



