1890.] WORMS OF THE GENUS PEIUCH.ETA. oO 



the feeble developmont of the gizzard. Thesubnervian vessel, com- 

 monly believed to be absent from Perichceta, is found, at least in some 

 species. The reprodui'tive organs, although not presenting any 

 Sf)ecially 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 Perichaetidse, as a wliole, is opposed to the view 

 that these are the most primitive Oligochseta ; while the structure of 

 the excretory system in my opinion favours the supposition. 



(4) The most striking evidence, however, in favour of the deri- 

 vation of tlie paired arrangement by a gradual reduction of a 

 continuous circle of setae, is aflForded by the structure of Deinodnlus. 

 Tliis genus is a native of New Zealand, and is in many respects 

 intermediate between Perichceta and Acanf.hodrilus. It is at present 

 the only Oligochaete known which possesses more than 8 setce in each 

 segment ^ and yet has not the continuous circle of setae of Perichceta. 

 Deitwclrilus 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. Deino- 

 drilus has dift'nse 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 corre- 

 sponding 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 

 tiiose of Acanthodrilus, 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. multi- 

 porus, beddardi, schlegelii, bilttikoferi, and untarcticus ; all these 

 species furthermore agree with Deinodrilus in having an incom- 

 plete prostomiuin (not dividing buccal lobe) and dorsal pores, 

 while ti)e 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. ?nultiporus. There are, 

 moreover, other intermediate forms. The question is really inti- 

 mately connected with the development of the nephridia ; if the 



' The statement that Hijpogason lias 9 seta' in each segment requires verifi- 

 catiou. 



