NEREIS. 155 
§ 10. Nereis rorata—The Leaf Nereis—Plate XX. Figs. 1, 
12, 18, 14, 15, 16, 17, 18. 
I have found great difficulty in identifying many species of the 
Nereids with those described by preceding authors. Perhaps certain 
points may not have struck me as sufficiently prominent, or I may have 
been unable, in a different course of investigation, from seizing the cha- 
racters specified by them. 
I acknowledge that when external characters seem sufficiently dis- 
tinet either for classification or recognition, it has not occurred to me as 
essential that the position of any animal, or its alliances, should be sought 
from internal organization. There seems no consistent reason why a 
horse should be killed and dissected to prove it a horse, or a dog to prove 
it a dog. Besides, destruction is the most fatal error wherein the zoolo- 
gist can plunge himself, for it 1s irredeemable. 
If I have committed mistakes in ignorance of previous observations, 
they can be easily corrected. All new names which I have employed 
are provisional, nor are any derived from obscure and equivocal etymo- 
logy. 
I do not affirm that this is the most philosophical, but it is the most 
convenient mode of imparting our knowledge to others. 
Length of the Leaf Nereis at least five inches ; breadth four lines. 
Head prolonged as a bare snout ; tentacula two, long and cartilaginous. 
Body slightly flattened, composed of many segments, with a row of pencils 
down each side ; likewise a row of thin flattened organs, like a blade or 
leaf, is reflected from each side on the back. This is their most perfect 
form. On the higher portion of the edge of the blade being uppermost 
and at the root of each, a prominent papilla with a pencil is displayed ; 
put in descending towards the posterior extremity, these blades dege- 
nerate into filaments. 
A small specimen or variety, extending an inch and a half, was ob- 
viously regenerating the posterior extremity. The tentacula were carried 
free before it when in motion, fig. 15; but at rest, they lay over the 
