| OCR Text |
Show 1890.] WORMS OF THE GENUS PERICH.ETA. 59 the feeble development of the gizzard. The subnervian vessel, commonly believed to be absent from Perichceta, is found, at least in some species. The reproductive organs, although not presenting any specially archaic characters, are not at any rate more modified than those of other Earthworms. In short it cannot be urged that the organization of the Perichaetidae, as a whole, is opposed to the view that these are the most primitive Oligochaeta ; while the structure of the excretory system in m y opinion favours the supposition. (4) The most striking evidence, however, in favour of the derivation of the paired arrangement by a gradual reduction of a continuous circle of setae, is afforded by the structure of Deinodrilus. This genus is a native of New Zealand, and is in many respects intermediate between Perichceta and Acanthodrilus. It is at present the only Oligochaete known which possesses more than 8 setcs in each segment1 and yet has not the continuous circle of setae of Perichceta. Deinodrilus has 12 setae in each segment, disposed at approximately equidistant intervals ; it therefore furnishes a connecting link between the continuous circle of setae and the paired setae. Deinodrilus has diffuse nephridia, more like those of certain species of Acanthodrilus than those of Perichceta ; the nephridia of a few of the anterior segments are more concentrated, as also are the corresponding nephridia of Trigaster lankesteri (Benham), a species which, in the opinion of Horst, should be referred to the genus Acanthodrilus, and which is at any rate closely allied to that genus ; this concentration reaches its extreme in A. multiporus, where the nephridia of these segments are metamorphosed into a gland opening into the buccal cavity. The reproductive organs are exactly like those of Acunthodrilus, but the clitellum, which occupies segments 14-1 6, and is developed continuously round the body, is like that of Perichceta. There is, therefore, a strong case for believing that Deinodrilus represents a stage in the evolution of Acanthodrilus from Perichceta, or of Perichceta from Acanthodrilus. The question is, which of these two alternatives is the more probable 1 The species of Acanthodrilus which come nearest to Deinodrilus are evidently those which have a diffuse nephridial system, i. e. A. multiporus, beddardi, schlegelii, bilttikoferi, and antarcticus ; all these species furthermore agree with Deinodrilus in having an incomplete prostomium (not dividing buccal lobe) and dorsal pores, while the first and last have the persistent double dorsal vessel of Deinodrilus. The species which are furthest away from Deinodrilus are such forms as A. dissimilis, where the prostomium completely divides the buccal segment, the nephridia are paired, and the dorsal pores have commenced to disappear. These extremes are connected by A. annectens, which has the incomplete prostomium and paired nephridia, but the anterior pair of nephridia are much specialized and open into the buccal cavity, as in A. multiporus. There are, moreover, other intermediate forms. The question is really intimately connected with the development of the nephridia ; if the 1 The statement that Hypogceon has 9 seta1 in each segment requires verification. |
