POLYXOE SCOLOPENDRINA. 393 



dorsal bristles is due to external influences, probably connected with commensalism. 

 The tip has evidently been destroyed, so that only the lower part of the bristle remains. 

 The stump, indeed, is often slightly thickened, and presents a furrow, giving the bifid 

 appearance already adverted to. In an example dredged off the Hebrides by Dr. Grwyn 

 Jeffreys the dorsal bristles are unusually complete, yet even in these the extreme tip 

 has disappeared and a probe-pointed end remains. This specimen shows, however, 

 that the normal condition of such a bristle is finely tapered to a filiform tip. The causes 

 of this remarkable change, which does not seem to affect the ventral bristles, may be the 

 secretions of the Terebella, or the friction of the bristles against the walls of the tube or 

 tunnel, the latter the most probable. 



The ventral division forms a short triangular lobe, the process for the spine being at 

 the apex. In some a bar of dark pigment occurs near the tip of the foot, just at the 

 base of the bifid region. Above the spine is the single large bristle, the shaft of which 

 is about twice the diameter of those adjoining. The tip is hastate and the point acute 

 (Plate XLI, fig. 5). As already indicated, this bristle is formed from a bifid one, the 

 secondary process disappearing as we proceed backward, and likewise the spines— traces 

 of which, however, even far backwards, are left in the faint transverse lines on the 

 dilated region. The relative proportion of the two kinds of bristles is shown in figs. 4 

 and 5. All the bristles which follow are bifid, with short spinous regions and moderately 

 stout shafts (Plate XLI, fig. 6, representing one of the superior forms with a longer tip, 

 and fig. 8 showing the tip more highly magnified). 



Toward the posterior end the large superior ventral bristle disappears. Before this 

 change takes place, however, two strong bristles occur on the foot as in front. The 

 dorsal tuft becomes minute posteriorly, and in the terminal feet the bristles are slender 

 and tapered — with very distinct spinous rows. 



The dorsal cirri, which in life are pale, extend in the preparations only a little beyond 

 the bristles, and gently taper to the slight swelling below the filiform tip. They have 

 numerous short cilia with clavate and truncate ends, and a few small cilia occur on the 

 base of the slender extremity. The ventral cirri are small and tapering, wdth similar 

 cilia. They do not reach the tip of the fleshy part of the foot. In some they are 

 rendered conspicuous by dark pigment at the base. 



Nerve-cords. — In transverse section nucleated cells are found externally and beneath 

 the cords, especially at the ganglionic enlargements. Strands from the exterior fibrous 

 layer also passed into the area of the cord in section, and in many such fibres seemed to 

 join the nucleated cells lying in the median line below the cords. 



Reproduction.— Females with well-developed eggs were found in the tube of Terebella 

 nebulosa at Herm in July, 1868, and in the chinks of rocks in tracks of Lysidice. In the 

 latter case males well advanced were also observed. 



Habits.— This form is more sluggish in its habits than Harmothoe imbricata or other 

 ordinary type. It is difficult also to conceive how the Terebella can be comfortable with 

 such a large commensalistic form in its tube, especially when the powerful bristles 

 of the Polynoe are considered. So far as observed it keeps its head towards the anterior 

 end of the tube, and thus its ordinary position is in agreement with that of the Terebella. 

 It is brittle, breaking into several pieces in lifting from one vessel to another. 



