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Show 312 MR. F. E. BFDDARD ON NEW OR [May 18, though there has never, to m y knowledge, been described so great a number of variations as I have been able to record in the present communication. Dr. Horstr has recorded a variation in the form of the spermathecae of Perichata indica and in the caeca of Perichceta musica, and Perrier2 in the spermathecae of Perichaita afjinis. Fourthly, and lastly, the probability, suggested above, of the occurrence of variations must not be left out of sight. Assuming it to be proved that an actual variation does occur in the present species, it will be necessary in the next place to eliminate those variations that are mere monstrosities, and that can hardly be considered to have any importance. Such are the occasional doubling of segments on one side of the body, as the variations Nos. 4, 12, and 14 ; these are comparable to such monstrosities among Vertebrata as two-headed lambs, calves with five limbs, and so forth, which are not in any sense reversions to an ancestral type, but are owing to some accidental cause, such as defective or excessive nutrition. On the other hand, the remaining variations are to m y mind of some importance. These will now be considered in some detail. It must be noted first of all that the variations occur in the generative system, and it is precisely the modifications of this system which have enabled systematists to classify the group. These variations affect all the parts of the generative system-the clitellum, the ovaries and their ducts, the spermathecae, and the male organs. I will commence with the clitellum. This organ and the relations which it bears to the male generative apertures has enabled M. Perrier to classify the whole group, after a fashion which is in the main satisfactory, though open to objections in certain cases. I have elsewhere urged that, in so far as it separates the Anticlitellians, i. e. Lumbricus and its allies, from the remaining genera of Earthworms, M . Perrier's system is by no means artificial, but bears out other anatomical differences. To distinguish the Intra- and Postclitellians from each other is not so easy a task : in the first place, we have genera like Megascolex, whose affinities are clearly with Perichceta, and which yet possess Intraclitellian generative apertures ; in the second place, we find that within the limits of a single genus, i. e. Acanthodrilus, the male generative orifices vary in position, and may be either intra-or post-clitellian. If the relations of the clitellum to the male generative apertures be used for classificatory purposes, it appears to me necessary somewhat to alter Perrier's definition, and to divide Earthworms into two groups, according as to whether the clitellum is placed far forward, and commences in front of the male generative orifices, or whether it is placed further back and commences behind the male generative orifices. That there is really a connection between the clitellum and the 1 Notes from the Leyden Museum, vol. v. p. 182. 2 Nouvelles Arch. &c. loc. cit. p. 106. These facts are of course liable to the same criticism as m y own. |