ror} A Commensal Annelid. 35 
Branchiz begin on the sixth somite, increase rapidly to equal half 
width of body, and diminish rapidly near posterior end of body ; 
each has a line of large ciliated cells, as also does the dorsal sur- 
face of each somite. Anus surrounded by about 14 papillz on 
each side,—a short posterior one, a much longer one, four shorter 
anterior, and then one still shorter. Dorsal sete long, acumi- 
nate (Fig. 3); ventral setae curved and with a flange (Fig. 5) till 
the twelfth somite is reached, then gradually replaced by an 
increasing number of forked sete (Fig. 4); setz of fifth somite six 
yellow hooks, each with a sharp flange on side near tip, and also a 
cluster of delicate hair-setæ. Found at Beaufort, N. C., living as a 
commensal in holes excavated in Gasteropod shells inhabited by 
Eupagurus pollicaris and overgrown by Hydractinia. Eggs laid in 
series of cases inside these dwellings; larve as in Figs. 12-14. 
Male thought to be much smaller than the above female form, 
and to live in the same shell. 
EXPLANATION OF FIGURES. 
Fig. 1.—Shell of Ilyanassa, cut open to show the openings into the 
columella and the built-up passage in the cavity of the shell on the left, all 
made by Polydora. x eight diameters. 
IG. 2.—General appearance of P. commensalis, dorsal view, preserved 
specimens. (Camera, Zeiss. 4 A). 
Fig. 3.—Ends of dorsal sete. (Camera, Zeiss. 4 F.) 
Fig. 4.—Forked ventral sete, with flange. (Camera, Zeiss. 4 F.) 
FIG. 5.—Pointed, anterior, ventral sete. (Camera, Zeiss. 4 F.) 
Fic. 6.—Setz of left side in fourth segment. (Camera, Zeiss. 2 D.) 
FIG. 7.—Anterior region of the body. (Camera, Zeiss. 2 A.) 
Fic. 8.—Posterior end of body. Paes, Zeiss. 2 A.) 
FIG. 9.—Diagram of cross-sections of body, showing dorsal openings ot 
nephridia, separate nerve-cords, ovary and free ova, digestive tract and 
blood-vessels, parapodia, muscles, and branchia. (Camera, Zeiss. 2 A.) 
FIG. 10.—Part of an egg-case, showing eggs in several chambers. (Cam. 
era, Zeiss. 2 A.) 
FIG. 11.—End of such a case, showing aborted chambers and cleaving 
eggs. (Camera, Zeiss. 2 D.) 
Fic. 12.Larva, ventral view. (Camera, Zeiss. 4 G.) 
_ Fig. 13.—Vertical, longitudinal, but not median section of Fig. 12. 
yeg Zeiss. 2 F. 
G. 14.—Advanced larva, showing provisional setæ and metamerically 
aes pigment blotches. (Camera, Zeiss. 4 G.) 
