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Show 1901.] OBTAINED DURING THE "SKKAT EXPEDITION." 83 to he seen at the periphery of the organ, the central part corresponding to the external depression consisting of rather less elongated cells. The considerable size of the organ as compared with the entire prostomium is apparent from the figure and is rather remarkable. It is probable, therefore, that a corresponding organ in other Tubificidae has not been overlooked. The setre are, as in the other species, all uncinate and without further complications ; there are no subsidiary hooklets between the two prongs in which the free extremity of the seta ends. There appear to be not more than four seta? to a bundle, and very often there are only two. In this species, as in other Oligochaeta, there would appear to be no setas upon the first segment of the body. However, iu longitudinal sections I was able to observe a small mass of muscles upon the first segment of the body, entirely similar in appearance to those which upon ensuing segments enwrap the setae, and corresponding exactly in position. The mass of muscular fibres was small, but I regard it as a vestige of the seta-bundles of that segment. Setae are certainly absent veutrally in the neighbourhood of the male pore-a feature in which the present species agrees with B. americanum, and differs from the European B. vejdovskyanum, in which there are specially modified genital setae of peculiar form replacing the ventral bundles. The lateral seta?, however, corresponding to the missing ventral seta? are present. Clitellum.-Bothrioneuron iris differs from its allies in the position of the clitellum, which is pushed back a segment and occupies segments xii. and xiii. In the middle of segment xii. lies the Male generative pore.-This aperture, single and median, is also a segment farther back than it is in B. vejdovskyanum and B. americanum. It is interesting to note that there is an apparent connection between the male pore and the clitellum in that an alteration in the position of one is accompanied by an alteration in the position of the other. The oviducal pores I have not seen. Spermathecalpores are not present. The absence of spermatheea? is one of the characters of the genus. Spermatophores.-Dr. Stole figures in B. vejdovskyanum a crowd of spermatophores attached to the body-wall round the male generative pore. In B. americanum I did not find these structures, though perhaps I was a little premature 1 in using their absence as a specific character. In three out of six mature examples of B. iris, I find a single spermatophore apiece also attached close to the male pore. The structure of these spermatophores is similar to those of B. vejdovskyanum. There is a thick stalk by which they are attached to the body-wall, which is of a yellowish colour. This stalk is merely attached to the epidermis superficially : it does not penetrate between the cells. Nor can I find any evidence of its 1 Monogr. Oligochseta, Oxford, 1895. The comparative raritylof the occurrence of spermatophores in B. iris may explain the failure to find them in B. americanum. They are probably distinctive of the genus. 6* |
