162 NEEEIS. 



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. 



