POLYCH.^TA. — BENHAM. 215 



maximum is retained till the 26th segment ; for the next 12 

 segments present five filaments ; the number then sinks to 

 4 (segments 39-70), to 3 (in segments 71-95), after which the 

 number decreases rapidly, and only the last three or four 

 segments are without gills. 



The gill as a whole has a very characteristic appearance, 

 indicated by the specific name — " thick-gilled " (PL xlii., 

 fig. 79) ; the gill filaments, whether few or many, are coarse, 

 arise fi'om the axis close together, and even in some cases 

 touching one another, and under a low-powered dissecting 

 lens the gill has some resemblance to a folded or wrinkled 

 membrane. 



There is a considerable range of variation in detail, to 

 which I refer later, but the above is generally typical, though 

 the maximum number of filaments may reach 8 or even 10 in 

 some individuals. 



The gill may be described as " small," whether the number 

 of filaments be few or many ; they only reach a short 

 distance up the much arched side of the body even when 

 placed upright against it. Usually they are not so directed 

 in these specimens ; they are ranged alongside the body, 

 usually with the free ends forwards. 



The parapodia do not present any special features ; they 

 are supported by two, or sometimes three, black acicula, and 

 on the 30th segment or thereabouts (for this point varies) an 

 inferior black bidentate acicular chcTeta appears, which is of 

 course continued to the end of the body. Under the micro- 

 scope this is seen to be a dark brown with golden margins, 

 though under a dissecting lens it is black and very con- 

 spicuous. The chaetae are illustrated by MTntosh, and need 

 no further description. 



The jaws are white below, the lower always, the upper 

 jawlets sometimes, but the denticulations are always white 

 tipped. On the right side the large dental plate (II) has six 

 teeth, of which the proximal and the distal are smaller than 

 their neighbours. Plate IV is hoodlike, with nine or ten 

 rounded denticulations ; plate V has a singled tooth. On the 

 left side, II has five teeth, IV is curved, has five, V has one, 

 and the unpaired plate (III) has seven teeth. 



The jaws, as is now well kno\vn, vary within limits as to 

 the precise number of denticulations of the plates, and 

 MTntosh gives instances. 



