NEREIS. 155 



10. NEREIS FOLIATA The Leaf Nereis. Plate XX. Figs. 11, 

 12, 13, 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- 

 tinct 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 is 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 ; 

 but 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 



