POLYCH^TA BENHAM. 41 



delicate hairs, extending beyond the tip, surrounding it and more or less concealing it. 

 Similar but less developed frills follow and occur along the greater part of the clueto 



(fig. 22). 



When studied under a higher power the chseta appears to be triangular or possibly 

 quadrangular in section (figs. 23, 24). Along two edges are ranged two series of spines 

 or teeth, which decrease in size as they are traced downwards proximally. Each pair 

 of teeth is connected across the " front " of the chaeta by a finely-striated membrane 

 or pectinated frill, the margin of which is comblike, as if it were made up of many very 

 delicate chitinous hairs closely set side by side. In the more distal of these combs the 

 hairs gradually increase in length, and beco ne flexible until the long hairs that form 

 the " beard " are produced. Similar but less developed frills extend outwards beyond 

 the teeth, down the " sides " of the chaetae, but in the proximal portion this lateral frill 

 x s replaced by a series of minute conical teeth (fig. 24). The spines or teeth are evidently 

 merely specialisations of the comb-teeth. 



The shorter upper chsetse of the bundle have simple frills, but as the chsetae get 

 longer the four or five of the distal frills become produced into the long hairs. There 

 is quite a gradual transition between the chaetao with simple frills and those with well- 

 developed " beards." 



These " beards," as Ehlers notes, entangle mud and debris, so that it is not 

 always possible to obtain a good view of the apex and to make out the real structure, 

 but in some of my mounts, both in Canada balsam and glycerine jelly, the apices are 

 fortunately free of mud and the structure is quite apparent. 



The ventral chsetse, too, are worthy of closer description than Ehlers has given 

 them. They are quite different from those of H . spinosa, as he has shown. 



Each presents two parallel series of short stout teeth or spines along the concave 

 edge, four or five in a series in the case of the longer chaetae. but reduced to three in the 

 smaller ones (figs. 25-27). The more distal spines in each series are simple and tooth- 

 like, but lower down each is seen to be surrounded at its base by a pectinated frill, or, 

 lower still, to be replaced by a frill whose edge, under a low power, has the appearance 

 of a tooth (fig. 27). These upper frills have quite a limited extent, but below them 

 come four to six closely-set small frills of very short pectinations which run right across 

 the chseta from side to side. 



The elytra on two of the individuals studied present marked variation from the 

 typical structure. In addition to the characteristic sub-marginal papillae, the elytra bear 

 conical and vesicular tubercles of brown colour. On the anterior elytra they are compara- 

 tively small, lying on the uncovered posterior region of the scale, but on the more pos- 

 teriorly situated elytra the tubercles become more conspicuous. They are here larger, 

 though less numerous, till, on the last six or seven, they are truly enormous vesicles (fig. 

 28). Whereas the tenth elytron carries some half dozen of these vesicles, the fourteenth 

 bears but one (fig. 29). These vesicles appear to be much enlarged and dilated tubercles 

 derived from the ordinary eohinulate tubercles characteristic of the species. 



83892 F 



