NATURAL HISTORY OF THE GENUS DEIIO, 



99 



they are embedded, except for a short distance at each end, in a 

 mass of large, clear, spheroidal cells. Wlien these masses are 

 not found on the segmental organs, they may occur on the septa 

 between the segments, or they may be absent altogether. I have, 

 however, been unable to discover the conditions which govern 

 their appearance. 



It has been stated that the internal end of the segmental 

 organs is a ciliated funnel ; they end externally in a somewhat 

 dilated portion, which communicates freely with the surrounding 

 element. Their function appears to be purely mechanical — 

 that of preventing undue distention of the body by the 

 fluid which passes through the wall of the intestine, and is 

 doubtless charged with effete material from the blood-vessela 

 which run in contact with it. Accordingly I find that, con- 

 trary to the general accepted opinion, the lumen of the tube of 

 the segmental organ is not ciliated, but that along one side of it 

 is attached a membrane whot^e undulations have the efi'ect of 

 driving the contained fluid in an outward direction. During full 

 activity it is not easy to see the edge of the membrane, though 

 the character of the movement even then is such as to suggest 

 some action quite diff"erent from that of cilia, and much more 

 nearly resembling that seen in the vibratile tags of the rotifers. 



When vitality is at a low ebb, it is by no means difficult, with 

 a power of about bOO diameters and suitable illumination, to see 

 the edge of the membrane. , The same conditions exist in the 

 case of other Oligochaeta, and indeed it is not easy to see how 

 cilia could act to advantage in a tube of such extremely small 

 calibre, nor am I aware of any instance in other forms of animal 

 life where such is the case *. 



The nervous system of Bero is excessively difficult to make 

 out, but appears to closely resemble that of Nais in its arrange- 

 ment. Perrier's account of the species which he observed 

 stops short at this point, his description being very meagre, and 

 in the few details given there is nothing to indicate any special 

 arrangement. 



* The observations which led to the above conclusions were made on 

 Tuhifex, Dero, Nais, Sfylaria, Chcetogaster, and Molosoma. In the case of 

 Chcetogaster only was there any difference. In the latter genus I liave never 

 seen any movement whatever in the tubular portion of the segmental organs, but 

 on one occasion observed in connection therewith an organ exactly resembling 

 the vibratile tags already referred to. 



