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Show 156 MR. F. E. BEDDARD ON A [Feb. 15, body, and there is therefore no prostomium ; in this character Thamnodrilus agrees with Urochata1 and Diachata2. It is a little difficult to distinguish the anterior segments of the body ; the buccal segment is divided externally by a very well-marked furrow, which appears to mark the line of division between two really distinct somites. A consideration of the number of the nephridia (see p. 160) leads me to infer that the divisional furrow does not imply a division into somites ; furthermore (see figs. 1 and 2) each of the two anterior rings is divided on each side by a longitudinal furrow, which corresponds in position with the dorsal pair of setae in the following segments ; the presence of this furrow is perhaps an additional argument in favour of regarding them as parts of the same somite. From the first segment to about the 8th, the breadth of the segments gradually increases; the longitudinal diameter of the segments also gradually increases up to the 8th or 9th, after which they become distinctly narrower. The segments of the clitellum are perfectly distinct, and are eleven in number. The clitellum commences with the 15th segment and terminates upon the 25th; very generally a trace of glandular modification is to be found upon the llth and 26th segments. The clitellum of Thamnodrilus is therefore nearly coextensive with that of Urobenvs, Urochata, Titanus, and Anteus. As in the former genus, the clitellum is not developed upon the ventral side of the body, but the extent of the area left free from glandular substance varies; in the anterior part of the clitellum, as far hack as segment 19, the ventral pair of setae as nearly as possible mark the boundary between the glandular and non-glandular portion of the integument; from the 20th segment to the end of the clitellum there is a space left between the glandular part of the integument and the setas. This is illustrated in the accompanying drawing (woodcut, fig. 2). The seta are disposed in pairs ; the distance separating the ventral pairs is 2*2 m m . in the clitellar region, the distance between the ventral and dorsal pair is 5'5 mm., while the dorsal pairs are separated by an interval of 12"5 m m . The setae are not remarkable in shape except upon the clitellum ; here they are modified and acquire the form illustrated in the drawing (woodcut, fig. 3) ; these peculiarly modified setae are apparently found throughout the clitellum (I have also found them in segments immediately anterior), where they replace both the dorsal and ventral pairs. The accompanying figure renders any minute description of these setae unnecessary, and will serve to show how exactly they resemble the clitellar setae of Urochata ; in this genus Perrier has recorded 3 the modification of the ventral pair of setae on the 20th segment; and Horst has stated i that in another species the ventral setas of four of the 1 Perrier, Arch, de Zool. Exper. t. iii. 2 Benham, Quart. Journ. Micr. Sci. 1886. 3 Arch. d. Zool. Exper. t. iii. (1874) p. 399, pl. xvii. fig. 52. 4 Midden Sumatra, Vermes, p. 8. |