308 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



worm, to build up a Memoir which shall give, as far as 

 possible, a full account of the structure of this form, 

 together with any other points of interest which may 

 arise during the investigation. 



This paper, previous to its publication, was 

 presented as a thesis for the D.Sc. degree of the University 

 of London, and in this connection I wish now to express 

 my gratitude to Professor Jackson who kindly photo- 

 graphed the drawings for me. It is with pleasure also 

 that I acknowledge my indebtedness to Professor Dendy 

 for valuable suggestions made while the work was in 

 progress and for careful criticism of the finished work, 

 which was carried out under his supervision in the 

 Zoological Laboratory at King's College. 



III. General Remarks. 



Tubifex rivulorum is found in large numbers in the 

 mud of the estuarine Thames, where, at low water, 

 they may often be seen as bright red masses. In every 

 consignment of mud which was examined this species was 

 always found associated with another worm — Limnodrilus 

 udekemianus — both belonging to the family Tubificidae. 

 The two worms can easily be distinguished from one 

 another if they are examined under a low power of the 

 microscope; but, with practice, it is also possible to 

 separate them when in the mass. One notices that the 

 anterior segments of all of them are reddish in colour, but 

 there is a marked difference in the posterior segments of 

 the two forms. In the case of Tubifex rivulorum the 

 posterior segments also are red in colour, but in 

 Limnodrilus udekemianus the red colour is masked by the 

 presence of pigment in the body wall. The pigment is 

 yellow or orange in colour, and is confined to a narrow 



