1892.] ON AQUATIC OLIGOCH^TOUS WORMS. 349 



5. On some Aquatic Oligochaetous Worms. By 

 Frank E. Beddard, M.A., F.R.S.E., &c. 



[ReceiTcd May 3, 1892.] 



In the following remarks I propose to bring together a few notes 

 upon certain aquatic Oligochseta which I have had the opportunity 

 of examining during the last year. 



i. On a Species of Dero. 



, Our principal knowledge of this genus is due to Perrier * and to 

 Stole *. A recent paper by Bousfield ^ is mainly devoted to discrim- 

 inating the species, though it contains a brief resume of the structure 

 of the genus Dero. 



I have recently been studying a species which I cannot identify 

 certainly with any known form ; my failure to identify it is largely 

 due to the fact that the differences in the vascular system of dif- 

 ferent species have not been worked out. Only in two, viz. D. per- 

 rieri and D. digitata, has the vascular system been described ; and 

 as these two show dissimilarities, it is at least possible that the 

 remaining species do also. In any case, the Dero at which I have . 

 worked differs from both these species. According to Perrier, 

 Dero perrieri has three pairs of contractile perivisceral trunks in 

 segments vi., vii., and viii. Behind the viiith segment the dorsal 

 vessel is not directly^united with the ventral. 



Of Dero digitata, Stole says, in the French abstract with which 

 his paper concludes, "II y a toujours deux vaisseaux lat^raux dans 

 chaque anneau suivant jusqu'au treizieme anneau (si I'animal est 

 completement ddveloppe). Dans les anneaux posterieurs les anses 

 vasculaires remplacent les vaisseaux lateraux." I cannot, I confess, 

 quite understand the distinction which is here drawn between the 

 two kinds of perivisceral trunks ; unless,^indeed, it is meant that 

 the anterior series are contractile. As Dr. Stoic's paper is in Bohe- 

 mian I am unable to say whether this is stated. 



The Dero examined by myself is a small species about a quarter 

 of an inch in length. Like other Deros it fabricates a tube, which 

 was always in the interior of half-decayed stems of plants ; the 

 stems had to be carefully torn up with needles to liberate the worms '. 

 The number of segments varied from 16 to 52. The characters of 

 the setae call for no comment ; the branchial processes most resemble 

 those of D. limosa. 



The vascular system is remarkable for the fact that there are six 

 pairs of contractile lateral vessels in segments vi.-xi. Those of 



^ " Histoire naturelle du Dero obtttsa^' Axch. Zool. Exp. t. i. p. 65. Bousfield 

 points out that the species investigated by Perrier is not Dero obfusa, but a new 

 form for which the name D. perrieri is suggested. 



2 " Dero digitata, O. F. MiiUer &c.," SB. bohm. Ges. 1885, p. 65. 



» " The Natural History of the Grenus Dero," J. Linn. Soc, Zool. vol. xx. p. 91. 



* Bousfield mentions this habit in D.furcata. 



Proc. Zool. Soc— 1892, No. XXV. 25 



