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Show 1893.J PROSTATE IN THE OLIGOCH.ETA. 485 Rosa's view appears to me the most probable; but this, however, he restricts to the Geoscolicidae. In certain Geoscolicidae, for example in Microchceta benhami, there are a series of paired glands in the neighbourhood of the male pores ; in some species, for example in Kynotus michaelsenii, setae are associated with these glands which are in all respects similar to the penial setae of other Earthworms. His view is that one pair of the glands have become converted into the " pseudoprostates" of other Geoscolicidae, which I regard, as already pointed out, as identical with the atria of other Oligochaeta. These glands are identical in structure with atria, and theoretical considerations also favour their homology. There are many instances among segmented animals of the reduction or concentration of metamerically repeated organs ; w e are more likely to be correct in assuming in such cases a reduction than a multiplication. It is easy on this assumption to understand the complete independence in the Acanthodrilidae of the sperm-ducts and the atria ; and, furthermore, the remains of additional glands such as occur in Bichogaster damonis; in this worm there are three pairs of tubular glands of which one only is connected with the sperm-duct. It is even possible that the glands (which I have called " capsulogenous") of the Perichaetidee are referable to the same category ; and it m a y be pointed out that the papillae upon which they open are sometimes regularly paired and correspond more or less accurately in position to the male pores. On this hypothesis of the original development of the atria out of copulatory glands, it is clear that those of the higher Oligochaeta are nearer to the primitive form of these glands than the lower; and, furthermore, that the retention of more than one pair and their independence Of the sperm-ducts are so far primitive characters. The Acanthodrilidae come obviously very near to the base of the series, though a little further off then certain of the Geoscolicidae. The Peri-chaetidse and the Cryptodrilidae can readily be derived from the Acanthodrilidae, and it must be remembered that some of the former have tubular atria and may therefore be nearer to the ground form. I do not propose in fact to discuss the relative positions of the different families, as I should have to take other characters into consideration with which this paper does not deal. The question of terminology has n o w to be considered; it evidently stands in need of revision. Are w e to retain the terms " atrium " and " prostate," or one or neither ? They have been used in so many senses that it will lead to misunderstanding if they are again used without careful definition. The use of the term atrium has the objections that M r . Benham has urged ; for in Acanthodrilus, for example, the atrium is not a chamber leading into any other chamber or duct; on the other hand, to speak of the " atrium" of Tubifex as a " prostate" would give a wrong impression, for the idea of a prostate is a glandular appendix to the male ducts. To speak of the sac itself as an atrium and of the coating of pear-shaped cells as the prostate is disadvantageous; for, excepting in Tubifex, the two parts of the terminal apparatus P R O C ZOOL. Soc-1893, N o . X X X I I I . 33 |
