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Show 1 9 0 4 .] EARTHWORMS FROM NEW ZEALAND. 261 the separation of Plutellus from these two genera, merely on account of the presence of meganephridia only depends on a point quite as small. I have already described a species of Plagiochceta, viz. PI. rossi, in which micronephridia replace the meganephridia of the other half-dozen species ; and embryology has taught us that the one is derivable from the other condition-that, at any rate in the genera Megascolides (Vejdovsky) and Megascolex (Bourne), the earlier meganephridium breaks up into numerous micronephridia. Is not the separation of the genera according to the condition of the excretory system a remnant of my own unfortunate attempt to classify the families of Earthworms into " Plectronephrica " and " Meganephrica " ? But without entering upon the laborious task of essaying to rearrange the " Oyptodriline " genera, I will express the opinion that a careful study of the form and structure of the prostate (together with other characters) may be more likely to lead us in the right direction. And, firstly, it seems to me profitable to distinguish the " tubular ''-cylindrical prostate, such as occurs in Megascolides ambialis, from the flatter, elongated " tongue-shaped " form of gland that occurs in Tokea, some species of Plutellus*, and others,and the " flattened, lobed, and compact" organ characteristic of such genera as Pheretimci and Megascolex. That these may form a developmental series, Miss Sweet's work (1900) has rendered probable; but they also appear to have structural differences that may turn out to be of diagnostic character. At any rate, they are easily recognised macroscopic characters; whereas the study of a worm from which the posterior end has been accidentally destroyed will not enable me to decide, in all cases, whether it belongs to the genus Megascolides or to Notoscolex as defined by Michaelsen. It is not difficult to imagine the way in which the genus Tokea has developed from an Acanthodriline stock, in which the cylindrical prostate, instead of being coiled compactly and confined to its proper segment, has burst away from this limited position, and elongating backwards has not only become flatter but has lost somewhat of its smooth external surface. Such a form of " tongue-shaped " gland occurs in Notiodrilus aucklandicus, in which each prostate extends through five or six segments. From such a form Rhododrilus may have developed-the posterior gland has disappeared, and the sperm-duct has shifted forwards so as to open close to the anterior gland ; whereas in Tokea the anterior gland appears to have gone, and the posterior gland to have moved forwards to meet the sperm-duct and to open externally in the 18th segment. * In looking through the descriptions of new species of Australian worms published by Spencer (Proc. Roy. Soc. Victoria in 1892, 1895, and 1900), I find that the only species that possess elongated " tongue-shaped " prostates (he calls them " tubular," but the figures show them to be like those of my species) extending through several segments belong to the meganephric genus Plutellus in Michaelsen's sense. |