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Show 1892.] ON AQUATIC OLIGOCH.ETOUS WORMS. 349 5. On some Aquatic Oligochaetous Worms. By FRANK E. BEDDARD, M.A., F.R.S.E., &c. [Received May 3, 1892.] In the following remarks I propose to bring together a few notes upon certain aquatic Oligochaeta which I have had the opportunity of examining during the last year. i. On a Species of Dero. v Our principal knowledge of this genus is due to Perrier 1 and to Stole2. A recent paper by Bousfield3 is mainly devoted to discriminating 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 ; m y failure to identify it is largely due to the fact that the differences in the vascular system of different 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 directlyvunited with the ventral. Of Dero digitata, Stole says, in the French abstract with which his paper concludes, "II y a toujours deux vaisseaux lateraux dans chaque anneau suivant jusqu'au treizieme anneau (si l'animal est completement deVeloppe). 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,vindeed, it is meant that the anterior series are contractile. As Dr. Stoic's paper is in Bohemian I a m 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 worms4. 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 1 " Histoire naturelle du Bero obtusa,' Arch. Zool. Exp. t. i. p. 65. Bousfield points out that the species investigated by Perrier is not Bero obtusa, but a new form for which the name B. perrieri is suggested. 2 " Bero digitata, O. F. Miiller &c," SB. bohm. Ges. 1885, p. 65. 3 " The Natural History of the Genus Bero," J. Linn. Soc, Zool. vol. xx. p. 91. 4 Bousfield mentions this habit in B. furcata. PROC. ZOOL. Soc-1892, No. XXV. 25 |