NATURAL HISTORY OF THE GENUS HE (10. 
99 
they are embedded, except for a short distance at each end, in a 
mass of large, clear, spheroidal cells. When 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-vessels 
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 whose undulations have the effect 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 different 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 diamefers and suitable illumination, to see 
the edge of the membrane. The same conditions exist in the 
case of other Oligochseta, 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 Dero 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 
Tubifex, Dero, Nais, Stylaria, Chmtog aster, and Molosoma. In the case of 
Chcetogaster only was there any difference. In the latter genus I have 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. 
