Branchionialdane vincenti 



149 



. Ch.Seg! 



Fig. r)9.— /}. viiiixnli. Anterior end, 

 dorsal aspect ; Pk. Prostomium, 

 liearinii groups of eyes ; Per. Peri- 

 stomiuni ; A.B.S. Achaetous body- 

 segment, CH.Sko.i. First cliaeti- 

 ferous segment. X 50. 



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

 wliicli vary a little in different specimens. The peristomium, which 

 is achaetous, is separated from the prosto- 

 luium by a shallow groove. There is no 

 delinite nuchal organ, that is, no pocket- 

 like invagination of the dorsal epithelium, 

 such as is present in late post-larval and in 

 adult examples of Arenicohi. The next 

 segment, whicli is also witliout chaetae, is 

 homologous with the acliaetous liody seg- 

 ment of Arcnicola (p. 87). Following this 

 are the chaetiferous segments and the 

 bluntly conical pygidium. 

 The number of chaeti- 

 ferous segments depends 

 on tlie 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 preserv^ed 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 Arcnicola (cf. 

 PI. XI, Figs. 33, 35). 



The annulation of the anterior and middle seg- 

 ments is mucli less delinite, and the number of aunuli 

 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 ^ rings. The last twelve to twenty segments 

 are generally bi-anniilate, the larger anterior ring 

 bearing the chaetae and the smaller posterior one 

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

 dently presented also by Langerhaus' specimens (see 

 his Fig. 2 1 (j), and it seems to be sufficiently constant 

 to be cited as one of the diagnostic features of this 

 worm. 



Chaetae. — Langerhans oViserved the presence of two kinds of 

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



' Some of these may not be true annuli, but maj- be due to foluing of the 

 bodv-wall brought about by contraction of its muscles. 



Fitr. 60.— />. vincenti. 

 The chaetae of one of 

 the posterior noto- 

 podia, in their natural 

 position with regar<l 

 to each other. 



