AND ZOOGEOGRAPHY OP INDIAN OLIGOCH^ETA. 121 



same genus. Michaelsen transferred the worm to Pontodrilus 

 (9), adding later (12) that it might be a Plutellus, an example of 

 convergence— though besides the primary features there were 

 others also which characterized both the new worm and the 

 previously known species of Pontodrilus. Ben ham (4) appears 

 to take the same view. Lastly, Michaelsen (11, p. 22) appears to 

 have definitely adopted the view that it is a Pontodrilus, 

 since he speaks of the apparent absence of Plutellus from New 

 Zealand. 



Some time ago I found an entirely terrestrial Pontodrilus in 

 material from the centre of Ceylon (19) ; this may, possibly, be 

 the ancestor of all the littoral forms (their littoral habit is of 

 course secondary) ; on the other hand, it may equally well be a 

 descendant of some one of the species of Plutellus which are 

 indigenous in Ceylon, while the bulk of the species of Pontodrilus 

 originated elsewhere. In any case, we seem to have a distinct 

 possibility — I will not say more — that worms which must, ana- 

 tomically, be placed in the genus Pontodrilus have arisen at 

 various times a.nd in various places. 



An extremely curious case is afforded by a genus newly 

 described by Michaelsen a.s Monogaster (15). It is essentially 

 a Dichogaster (subfam. Trigastrinse) in which the two gizzards 

 have, as it were, run together again, probably in consequence of 

 the disappearance of the septum between them. In the evolution 

 of Dichogaster the steps from the original Notiodrilus ancestor 

 have been as follows : — First the doubling of the gizzard, then the 

 development of the micronephridial condition, and then the 

 development of calciferous glands in certain postgenital segments. 

 In Monagaster, therefore, the gizzards having secondarily united, 

 the essential characters are the micronephridial condition, the 

 calciferous glands, and a single gizzard. But these are exactly 

 the characters of Oetochcetus, which belongs to an altogether 

 different subfamily, the Octochsetinse. In this line the initial 

 change was the breaking up of the nephriclia, and this has been 

 followed in Oetochcetus by the development of calciferous glands, 

 here too, as in Dichogaster and Monogaster, in the segments behind 

 the ovaries; the gizzard has never been double. There is nothing 

 in the arrangement of the male organs to distinguish Monogaster 

 from Oetochcetus; the calciferous gla.nds in Monogaster are three 

 pairs, in segments xv., xvi. and xvii., while in Oetochcetus they 

 are one or two pairs, in xv., xvi., or both : but this could not be 

 a ground for generic distinction. There is a difference in type 

 between the inicronephridia in the two genera, — numerous and 

 tubular in Oetochcetus, fewer and saclike in Monogaster ; but 

 beyond this the only distinction is in the distribution — Mono- 

 gaster comes from Africa, from the Dichogaster region, while 

 Oetochcetus has never been found farther west than the Malabar 

 coast of India. The line of descent of each is perfectly plain ; 

 still the case illustrates my contention, that the same end may 



