272 DISCOVERY REPORTS 



Observations. In his original description Beddard referred this species to Acantho- 

 driliis georgianus of Michaelsen. In the same year Michaelsen^ re-described ^. ^^or^m«M5 

 and pointed out that Beddard's specimens from the Falkland Islands differed in several 

 respects from this species. Michaelsen suggested that Beddard's specimens were not 

 A. georgianus, but were identical with A. bovei of Rosa. Beddard later (1893) accepted 

 Michaelsen 's first suggestion and referred these specimens to a new species, A. falc- 

 landicus. According to these authors M. falclandiciis diff"ers from M. georgianus in the 

 following characters: 



(i) The intersetal distance cd is markedly greater than ab. 



(2) The nephridiopores are in setal line c, not slightly below it. 



(3) The gizzard is rudimentary, not absent. 



(4) The penial setae are different. 



As regards the first character new material of M. georgianus described in the present 

 investigation shows that the intersetal ratios are subject to great variation and may 

 frequently be the same as in specimens of M. falclandicus. The nephridial pores of 

 M. falclandicus, though never as markedly below the setal line c as in M. georgianus, are 

 usually slightly so, and this character is therefore unreliable for diagnosis. The difference 

 between a rudimentary gizzard and one which is " totally absent" is also very deceptive 

 as a diagnostic character. The only character which can be relied on in the separation of 

 these species is therefore that of the penial setae. Since the earlier descriptions were 

 incomplete the penial setae of the two species are re-described and figured in the present 

 communication. In both species the setae of the a and b bundles are of about the same 

 dimensions, but differ in their ornamentation. The b setae of the two species are very 

 similar ; those of M. georgianus are shorter and slightly thinner, with ornamentation of 

 the same type as in M. falclandictis, but the teeth fewer and further apart. It is in the 

 a setae that the difference between the two species is most readily appreciated. As with 

 the b setae, those of M. georgianus are shorter and somewhat more slender than those of 

 M. falclandicus ; in both species the distal end is slightly coiled through one revolution 

 of a spiral and is ornamented with a spiral row of knobs ; in M. georgianus these knobs 

 are few in number (three or four), and either smooth or with a slightly serrated edge ; in 

 M. falclandicus there are a large number of knobs (ca. thirteen) and these are prominently 

 scalloped ; in M. georgianus the direction of the spiral is anti-clockwise on the right and 

 clockwise on the left, in M. falclandicus the reverse is the case. 



Both these species are also very close to M. aquarumdulcium and M. anderssoni, but 

 may be distinguished by the form of the penial setae. In the two latter species (if they 

 are not identical) the a setae are considerably more slender than the b and show no spiral 

 ornamentation. 



1 Michaelsen, V^.,Jahrb. d. Hamb. Wtssensch. Anst., vii, 1889, p. 57. 



