AND ZOOGEOGRAPHY OF INDIAN OLIGOCHiETA. 129 



reduplication of the gizzard. From Trigaster is derived Dicho- 

 gaster, in which calciferous glands are developed in segments 

 xv.-xvii. or thereabouts. 



The geographical relations of this subfamily are qnite different 

 from those of the preceding groups. Diplocardia is found in 

 North and Central America, and its descendant Trigaster in 

 Central America and the West Indies. Dichogaster is endemic 

 in Central America and the West Indies, and also in tropical 

 Africa; all the species that are found in India are introduced, 

 with the possible exception of one only. EudicJiogaster, a purely 

 Indian genus, is derived from Trigaster by Michaelsen (by the 

 development of calciferous glands in segments x.-xii. or there- 

 abouts) ; but in a previous section (Xo. I. of the present series) 

 I have given my reasons for believing that this genus belongs to 

 the Octoch8etina3. 



The view of Michaelsen is that Trigaster spread from its 

 original home in Central America and the West Indies by means 

 of a land-bridge to Africa, and thence, bj^ a land-bridge in the 

 Pliocene, to India, where it gave rise to Etidichog aster, itself 

 disappearing in India in the transformation. Dichogaster had 

 its origin from Trigaster on the American side of the Atlantic, 

 crossed the Atlantic by the same bridge as T^-igaster and reached 

 Africa ; its indigenous range at present extends no further — 

 indeed it does not seem to have as yet quite reached the eastern 

 shores of Africa (though a large number of peregrine species 

 are known from farther east* including India). Trigaster has 

 been exterminated in Africa by the dominant genera Eii,drilus 

 and its own descendant Dichogaster, which between them quite 

 dominate this region, in the same way that so many genera of 

 Megascolecidse ha\"e disappeared from the Malay Archipelago in 

 consequence of the sprend of Fheretima. 



(d) The Aloniligastridce. This fnmily consists of only a few 

 genern. Without going into the relationships of these, it m&j 

 briefly be stared that Desmogaster, the supposed ancestral genus^ 

 is found in Borneo, Sumati'a, and Lower Burma, and its descen- 

 dant Ei'.poly gaster has a similar distribution. Draioida, the 

 largest genus of the family, is predominantly S. Indian (though 

 its range has recently been shown to be more extensive than was 

 believed) ; Moniligaster, a small genus very close to Dratvida, 

 belongs to the same region. 



Michaelsen supposes that S. India and Ceylon were peopled by 

 this family by means of a land-bridge across the Bay of Bengal, 

 and rejects the supposition that the forerimners of the present 

 S. Indinn Moniligastrids could have travelled by land I'ound the 

 head of the Bay ; they would have left some ti-ace of their passage 

 in that region (a number of endemic species of Draicida have, in 

 fact, been recently shown to inhabit this region). Besides, the 

 bridge was in existence when the Megascolecinae passed over 

 to S. India, and so was available for the Moniiigastridse too. 



Proc. Zool. Soc— 1921, No. IX. 9 



