Branchioinalda ne vinccnd 



149 



Fig. 59. B. vincenti. Anterior end, 

 dorsal aspect ; PR. Prostomium, 

 bearing groups of eyes ; PER. Peri- 

 stomium ; A. B.S. Achaetous body- 

 segment, CH.SKG.I. First chaeti- 

 ferous segment. X 50. 



laterally groups of eyes (Fig. 59), the number and disposition of 

 which vary a little in different specimens. The peristomiuin, which 

 is achaetous, is separated from the prosto- 

 miuni by a shallow groove. There is no 

 definite nuchal organ, that is, no pocket- 

 like invagiuation of the dorsal epithelium, 

 such as is present in late post-larval and in 

 adult examples of Arenicola. The next 

 segment, which is also without chaetae, is 

 homologous with the achaetous body seg- 

 ment of Arenicola (p. 37). Following this 

 are the chaetiferous segments and the 

 / bluntly conical pygidium. 



The number of chaeti- 

 ferous segments depends 

 on the stage of growth 



attained. The largest number observed is fifty-one. 

 Each chaetiferous segment bears notopodial 

 chaetae and neuropodial crotchets. The notopodia 

 and neuropodia of all the preserved specimens are 

 only slightly elevated above the rest of the body 

 wall. The neuropodia are short, being much shorter 

 than those of post-larval ecaudate Arenicola (cf. 

 PL XI, Figs. 33, 35). 



The annulation of the anterior and middle seg- 

 ments is much less definite, and the number of annul! 

 per segment less constant than in Arenicola. In the 

 larger specimens the segments from the third or 

 fourth to nearly the thirtieth are divided into four to 

 seven 1 rings. The last twelve to twenty segments 

 are generally bi-annulate, the larger anterior ring 

 bearing the chaetae and the smaller posterior one 

 the gills (PL XI, Fig. 33). This condition was evi- 

 -B. vincenti. dently presented also by Langerhans' specimens (see 



Fi g 



Interior 'noto* his Fig. 21 g), and it seems to be sufficiently constant 

 & to be cited as one of the diagnostic features of this 

 worm. 



to each other. 



CHAETAE. Langerhans observed the presence of two kinds of 

 notopodial chaetae, which he figured, but not very accurately. 



1 Some of these may not be true annuli, but may be due to folding of the 

 body-wall brought about by contraction of its muscles. 



