polych.*;ta fkom north America. 791 



The full-grown branchifB had about 17 pairs of ciliated pinnfe, 

 which stopped short some distance from the top of the rachis 

 leaving a fairly long filamentary end. In sections the rachises 

 are seen to be strengthened externally by a thick layer of chitin, 

 which also forms a protecting layer round the pedicle (fig. 6 d), 

 and is continuous over the operculum with the thick hoi"ny plate 

 on its top. This plate was often covered with sand, and many 

 specimens had infusorians, etc. attached to it. The central 

 coelomic space in the pedicle is lined with peritoneum provided 

 with very large conspicuous nuclei. There is a small vessel 

 running along its whole length and enlai-ging in the opercular 

 cup into a spherical vesicle (fig. 6 e). This is filled with a finely 

 granular precipitate, and from its wall and general appearance 

 seems to correspond with the branchial blood-vessels of the 

 ordinary gill rachises, though I do not see that it can have any 

 respiratory function. It is apparently suspended in a fine 

 reticulate connective-tissue which easily s]ninl«5 away from the 

 epithelial cells ; Levinsen, in his original description (10. p. 203), 

 suggested that it might be a new operculum forming in the old 

 one. 



The collar is very wide — the entire ventral lobe being generally 

 reflexed — the latero-dorsal lobes are continued down the dorsal 

 side to between the second and third thoracic setae, where they end 

 abruptly, giving the appearance of a short thoracic membrane. 

 This was a constant characteristic in both small and large 

 specimens. I have not seen it i-eferred to before, but do not 

 think it necessary on that account to separate this as another 

 species. 



With the lai-ge collar setse (fig. 6 b) are a few shorter curved 

 forms with very narrow blades. 



The abdominal setae agree with those described by St.-Joseph 

 (24) for the genus (fig. 6 c). 



The abdomen is long and slightly dorso-ventrally flattened, 

 with 25-40 segments. The tori contain about 17 uncini, but 

 there are only a few setae to each segment. The dorsal longi- 

 tudinal muscles are grea,tly developed — there is a small fascicle 

 of ventral ones on each side of the wide faecal groove. The large 

 ventral nerve-cords are separated from one another. The 

 epithelium consists of low columnar cells with numerous gland- 

 cells containing the usual spherical masses which stain easily 

 with iron hsematoxylin. 



This species seems to agree very closely with that described 

 by Bush (3) as Hyalo'pomatopsis occidentalis. I cannot under- 

 stand why this form with no thoracic membra,ne and " trumpet- 

 shaped " abdominal setae is put in a genus, of which St.-Joseph 

 writes (24. p. 264) : "II m'a fallu creer un genre nouveau 

 Hycdojjoinatopsis pour le Hycdopomatus langerhansi Ehl. et le 

 //. marenzelleri Lang., qui par la presence d'une membrane 

 thoracique . . . ne pouvaient rentrer, comme I'avait du reste 

 prevu Langerhans, dans le genre Hyalopomatus tel que I'avait 



o3* 



