No. 415.] NEREIS VIRENS SARS. 565 
time brings out the structural conditions of the internal organs 
in the abnormal region. In the sixth abnormal metamere 
(Fig. 2) the two pairs of appendages are arranged symmetri- 
cally, dx. and s. being lateral and in normal position, while 
s. and Zx., placed back to back, project downward, or in a 
normal ventral direction. But there is apparently a well- 
developed ventral region on either side of these parapodia 
(s. and Zx.'. At first glance it is difficult to tell which is the 
normal ventral side and which the interpolated part. In each 
Fic. 2. — View of the anterior face tic) of ti of the sixth 
abnormal somite. (X 6.) cd.n., err , nerve “cords; dx., right normal parapodium ; Zx.', 
right abnormal parapodium ; u.l., sme, longitudina inal ventral — ; s., left normal 
parapodium; s.’, left ssim ptrapodiüm; va.v., va.v., ventral vessels. 
region there is a well-formed nerve cord (ca.n., ca.n.'), flanked 
by a pair of longitudinal muscles (mu./., mu./.'). The slightly 
smaller size of the parts on the right of the figure (the left side 
of the worm) is the main indication that this portion has been 
interpolated. 
On examining the parapodia of this somite more minutely, 
find that they are all normal in every m Indeed, zx. and 
s.' might be taken for the normal pair, if s.' and dx.’ were not 
slightly smaller than s. and dr. That these latter, however, 
are the normal appendages is plain from the conditions found 
