XERILLA AN ARCHIANNELID. 
399 
ventral surface of the body. It proceeds evenly along/ 
with passive parapodia, only twisting its head from side to side 
and using the prostomial and first pair of parapodial cirri 
as feelers. In this way it progresses at a fair speed. 
Occasionally, however, and especially when disturbed, it 
may rise up from the bottom swimming, or rather darting, 
swiftly -for a short distance with a sudden rapid sinuous 
motion of the body, as already noticed by de Quatrefages. 
Alighting on some new spot the worm will then continue its 
even course propelled by cilia alone. The parapodia are 
used chiefly when the worm is climbing over a rough surface. 
When placed in a shallow dish of sea-water Nerilla may often 
be seen floating near the top close against the surface film ; 
if disturbed it darts away to the bottom. 
Nerilla has an aversion to light ; it is negatively photo- 
tactic. Thanks to this property I was able to devise an easy 
method of collecting large numbers from the mud in which 
they live. The method is as follows : A flat dish with about 
an inch of sea- water is placed near a window, and in it is 
gently dropped some mud at the end near the light. In a short 
time a solemn procession of the little worms is seen issuing 
from the mud and moving towards the darker end of the 
dish. Here they gather, and can then be easily taken up in 
a pipette. 
They are very hardy ; it is easy to keep them alive in 
ordinary sea- water for several days even in quite a small 
capsule. In filtered sea-water free from bacteria, but to which 
have been added some of the algse on which they feed, they will 
thrive. I have a large number alive and breeding in Oxford, 
which were kindly sent to me from Naples by post last July 
by Mr. K. R. Lewin. 
External Morphology. 
The body consists of a prostomium, nine trunk segments, 
and a pygidium (fig. 1). Two pairs of eyes are placed on 
the dorsal surface of the prostomium, and in front of these 
