Aquatic Oligochaeta from Japan and China. 
87 
in other members of the genus and subfamily, it appears to be entirely wanting here; 
the examination of a series of microscopic sections might perhaps have given some 
indication of a thickening of the oesophageal wall in one or other segment, but with 
only two specimens available I did not undertake this. Mud, being softer and in a 
finer state of division than earth, needs less grinding up; hence, supposing limnic 
Megadrili to have originated from terrestrial forms, the gizzard would regress from 
disuse in the former. So Michaelsen, on the Geoscolecid-Lumbricid group in general 
(9): — " Einzelne Abtheilungen . . . . rein limnisch sind und, wahrscheinlich in Folge 
dieser Lebensweise, gewisse Organe . . . . zuriickgebildet oder ganz abortirt zeigen. 
Diese Riickbildung betrifft besonders den Muskelmagen und manchmal auch die 
Samentaschen. " And while the coarser sediment is deposited in the shallower water 
near the shores, it is only the finest particles that are carried out to the depths ; 
hence the deeper the habitat, the more vestigial we should expect to find the 
gizzard. 
(4) Ivimnic forms have not as a rule much zoogeographical value. Thus the 
genus Chaetogaster is cosmopolitan. 
Michaelsen (13) supposed that Braiichiiira sowerbyi was possibly originally an 
inhabitant of the tropics, since when he wrote the worm had been found only in arti- 
ficially warmed water in Botanical Gardens. But it has since been found in natural 
surroundings in France, and in extratropical India where hoarfro.st lies on the ground 
in the mornings for some weeks in the year, as well as now in China and Japan; and 
one of the latter countries has perhaps as good a claim as any other to be considered 
as its place of origin, since Kawamuria, — a close relative, or even, as I have argued 
above, possibly its immediate ancestor, — lives also in Japan. It is at any rate a 
curious coincidence that the two worms should occur in the same small collection of 
only half a dozen identifiable forms. But it is unwise to lay stress on isolated facts 
of this kind; a related genus, Bothrioneiirum, is represented by one species in the 
Malay Peninsula, one in S. America, and one in Austria; and both Kawarmiria and 
Branchiiira may have a wider distribution than appears at present. 
Species of Criodrilus are known from S. America and Costa Rica, and the genus 
also includes the fairly common C. lacuum, found in Central Europe and extending 
to Syria and Palestine and doubtfully also to India. No species of Criodrilus how- 
ever was found in Lake Baikal, or in the Teleckoe Lake in the Northern Altai 
(Michaelsen, 10) ; and it is impossible to say whether the forerunners of C. hathyhates 
reached Japan from the direction of America or from that of Europe, or whether the 
present species is a relic, — the area of distribution of the genus Criodrilus having 
perhaps been at one time circummundane. Michaelsen looks on the Geoscolecidae as 
having had previously a more extensive distribution, the terrestrial members having 
(in the Old World) given way before the younger branches of the Megascolecid tree, 
and especially before the expanding Lumbricidae (Michaelsen, 9). Only the hmnic 
divisions of the family have been able to maintain themselves in the territory which 
has been appropriated by their younger rivals. A depth of 180 ft. may be supposed 
to have removed the present species out of the region of competition. 
