ON AXNELIDA. 97 



Leaving systematic distribution, however, for the present, we 

 must now give a succinct view of the structure and history of 

 the nereides. 



The body of the nereides is in general extremely elongated, 

 slender, attenuated, cylindrical, sub-cylindrical, or even 

 sometimes depressed, especially undenieath. Finally, a little 

 broader in the middle, it becomes gradually attenuated to- 

 wards the extremities, but much less so towards the anterior 

 extremity, which is altvays more or less truncated, than to- 

 wards the posterior. It is formed of a great number of rings, 

 or segments, broader than long, very distinct, very mobile, 

 one upon the other, in their whole circumference, and the 

 broadest and longest of them are in general towards the an- 

 terior third of the body. The posterior ones decrease insen- 

 sibly as far as the last. On the first or second anterior ring 

 we may pretty frequently distinguish one or two pairs of 

 black orbicular spots, which have been regarded as eyes. 

 These first rings, in certain species, do not appear to be com- 

 plete, but rather cleft, or open in their inferior part, so that 

 the aperture of the oesophagus is preceded by an oblique in- 

 ferior cleft, to which the name of mouth has been given. In 

 other species these rings are complete, but they are wide 

 enough to permit the re-entrance of a considerable cephaloid 

 proboscis ; which itself is sometimes composed of one or two 

 rings. 



To suit to this disposition of the cephalous rings, it is 

 evident that the mouth must present different forms. In the 

 first case, it is a cleft more or less long ; in the second, it is a 

 round hole, sometimes accompanied in its circumference with 

 papillae, or papillary tubercles disposed in radii. 



The posterior extremity of the body of the nereides ter- 

 minates by being very much flatted. Sometimes the last ring 

 is in the form of a dagger point, which alone extends beyond 

 the anus. This last is always very broad and transverse. 



VOL. XTII. H 



