162 NEREIS. 
without shifting. On sudden agitation the whole body seems in a blaze, 
while still stationary in its own element and preserving the same position. 
But beautiful flashes are emitted, when the vessel, having been emptied, 
is replenished with fresh-water ; and should different parts of the sides 
be occupied by several specimens, as is always the case when collections 
are recently introduced, so many brilliant corruscations will escape from 
all the parts of the vessel where they are established. This interesting, 
though rude experiment, may be repeated several times with the same 
subjects. However, if these are to be preserved, the vessel must be 
speedily drained, as few marine animals can survive even a short humec- 
tation or immersion in fresh water. 
Several species of the Nereis swim by horizontal, serpentine un- 
dulations, the body assuming the figure of what is vernacularly called a 
flaming sword, which, in an animated being, appears very singular to the 
beholder. 
I have purposely abstained from specifying the number of segments 
as a leading feature. 
It appears to me in as far as hitherto determined, that a frequent, if 
not a continual evolution of new articulations is advancing, so as to pre- 
clude calculation of any precise number as belonging to a species. There 
is also considerable difficulty in estimating the exact complement of the 
antennular organs. 
