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Show 308 MR. F. E. BEDDARD ON NEW OR [May 18, 5. REMARKS ON THE VARIATION OF PERIONYX EXCAVATUS, E. Perrier. Hardly anything is at present known with respect to the variations in structure which may occur in a given species of Earthworm; and in order clearly to define the limits of different species it is evidently a matter of some importance to ascertain how far variation may take place. The description of by far the majority of exotic forms of Lumbricidae has depended upon the dissection of a very few examples, so that many of these descriptions must be qualified by admitting the possibility that they relate only to what may be termed for convenience' sake the normal conditions of structure. Such a criticism, however, can only be applied to those instances in which a species or genus has been created for the reception of a single individual, which may show well-marked divergencies in structure from its immediate allies ; if a number of individuals agree to differ from a second series of individuals in certain well-marked characters, it would be obviously necessary to separate the two groups either generically or specifically as the case demands. The Lumbricidae are a group which exhibit a most remarkable variability in internal structure, more especially of the generative system ; in accordance with this variability they have been divided into a considerable number of species and genera. It might well be expected that this group, which is apparently universally distributed and is at present no doubt as abundant, or even more abundant1, in individuals as well as in species as it ever was, is still in course of differentiation into new forms ; any accidental variation may be the first term of a series which will ultimately lead to the formation of a new species. I have had the opportunity of examining, through the kindness or my friend Mr. Herbert Barwell, rather more than 400 individuals of a Philippine Earthworm belonging to the genus Perionyx ; this worm exhibits a number of variations which appear to me to be really variations, and not marks of specific distinctness. The reasons for this belief will be stated after the facts have been detailed. The Earthworm in question appears to be identical in every respect with Perionyx excavatus: it differs in no point from M . Perrier's2 description of that species. I need hardly therefore describe in detail its specific characters, as it would be merely repeating what Perrier has already said ; it will be necessary, however, briefly to indicate the main features of its organization in order to render clear what follows. The setae are disposed in a continuous row round the middle of each segment; they are not disposed upon a ridge as in Perich&ta, 1 There seems to be a certain relation between the abundance of Earthworms and the cultivation of the soil; this fact is noted in a short article on Earthworms in the ' Field' of March 27, 1886. M y friend Mr. James Cavan informs me that in California fishermen know well that if worms are required for bait they must be sought for in cultiyated land. 2 Nouv. Arch. d. Mus. t. viii. (1872) p. 126. |
