68 AUSTRALASIAN ANTARCTIC EXPEDITION. 



nuchal organ of many Polychaeta. Just anterior to this, but only visible from the 

 ventral surface, is a little patch of pigment a short distance from the anterior margin 

 of the prostomial plate; this probably represents an eye-spot (fig. 113). 



The prostomial plate is continuous dorsally and laterally with a curved semi- 

 circular ridge, from which it is separated by a shallow furrow, but this ridge is also 

 continued downwards across the ventral surface to form the lower lip, which is closely 

 pressed against the upper lip formed by the prostomial plate, the anterior edge of which 

 it does not reach (fig. 109). 



Whether one is to regard this curved ridge as the hinder region of the prostomium, 

 such as is described for several Ampharetids, or as the peristomium, seems uncertain; 

 but from its relations I take the latter view. 



The ventral region of this peristomium is produced forwards in the middle line, 

 so that a median and two lateral regions of the lower lip are distinctly marked off from 

 one another ( fig. 113); the median lobe has a straight transverse edge which is abruptly 

 limited on each side by a nearly longitudinal margin, that turns sharply outwards to 

 form the anterior edge of the lateral region of the lip. When the animal is seen from 

 above, this lower lip is invisible, since it is overhung by the prostomial plate, and 

 even when viewed from below, its base is partly concealed by the forward extension of 

 the ventral surface of the following segment. 



The branchiferous segment, the second of the body, is very much compressed 

 on its dorsal portion, so as to form an upstanding fold; its ventral portion is thick and 

 glandular and conceals in great part the lower lip. 



On the dorsal surface this segment is rather longer than the peristomium, and 

 carries a pair of admedian, upstanding gills of unusual form and structure (fig. 109). 

 Each gill is 4 mm. in height, i.e., about half the height of the body at this point; it 

 consists of a rather thick axis, which bears along its whole length four undulating 

 tough membranes, two on the external and two along its internal or medial surface. 

 The membranes are broad below and taper distally so that the form of each gill may 

 be described as quadrifoliaceous and lanceolate, in Grube's terms. 



The two gills are connected at their bases by a low transverse membraneous 

 ridge (perhaps exaggerated by the contraction of the body), which is continued outwards 

 and downwards almost to the level of the notopods of the following segments. 



Passing backwards and outwards from this ridge, commencing behind each 

 gill, is a deep channel, bounded by a couple of narrow walls which, about midway 

 in their course towards the base of the third notopod, unite above the channel and 

 convert it into a tunnel. This tunnel appears to end blindly (fig. 110). 



Two quite similar but successively shorter structures pass from the branchiferous 

 ridge towards the second and first notopod, above which they respectively terminate. 



