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Show 60 MR. F. E. BEDDARD ON [Feb. 4, presence of a single pair of these organs in each segment is the archaic condition, then Perichceta will be a modification of Acanthodrilus ; but this view is confronted with two serious difficulties-(1) the apparent specialization of a part of the nephridial network to form a series of paired nephridia in Megascolex armata and in Megascolides australis (Spencer) will require explanation; and (2) the connection of the vasa def'erentia with the atria ( = prostates) will have to be regarded as having been derived from a condition in which these organs are independent of each other (Acanthodrilus). These structural peculiarities are capable of an intelligible explanation if we assume that Deinodrilus is an intermediate stage iu the evolution of Acanthodrilus from Perichceta. The remarkable arrangement of the setae in certain Australian Perichcetce, which I have in the present paper associated together in the genus Anisochostu, and the commencing reduction of the setae in P. indica must be considered in relation to this question. It is noteworthy that in these cases it is the anterior segments only which differ from the posterior in the reduction of the setae. The formation of a "head" is also brought about by specialization in the alimentary and excretory systems, and by a partial obliteration of the ccelom and loss of internal segmentation. These facts tend to show that the reduction in the number of the setae is also secondary; and this reduction is very general in Perichaetidae, though not anywhere so apparent as in Anisochceta. It is true that, as Perrier first pointed out, the hindermost segments of Perichceta may also show a reduction in the number of setae; but this fact may be in accord with the views here advocated, inasmuch as the nephridial system in Megascolides begins to be specialized in the posterior region of the body. I would, however, rather insist upon the increase in length produced by the addition of new segments at the end of the body, and explain the few setae of these segments as due to their recent formation and consequent imperfect development. If we were acquainted with a species of Lumbricus in which the anterior segments were provided with a larger number of setae than ordinarily, it would certainly be set down to " cephalization ; " there is therefore nothing unreasonable in regarding the converse change, which actually occurs, as due to the same cause. These facts, therefore, are at least not contrary to the assumption that the "perichcetous " condition is the more primitive. Among the species of Anisochceta which show the reduction to 8 setae per segment, some have more segments modified in this way than others ; there is, therefore, evidence of a gradual change in this direction which lends more weight to the arguments here advanced than if all were modified to exactly the same extent. In the latter case the facts could be referred only to a modification affecting the " head" and comparable for example to the loss of the setae in some of the first few segments in Chcelogaster; as it is the facts appear to point to a gradually advancing reduction of the setae commencing in the most modified region of the body. |