ITS TENACITY OF LIFE. 155 



other Dwsibranchiate Annelidans, is divided into a 

 consecutive series of rings. Upon either side of each 

 ring is situated a singular appendage, which acts as 

 a gill or branchial organ, by the exercise of which the 

 blood of the animal is effectually purified, and respira- 

 tion adequately provided for. 



When the Nereis is in a state of repose, these gills 

 are laid flat over its back ; but in a state of activity 

 they are fully spread out, and act as * paddles,' by 

 aid of which the animal is enabled to glide through 

 its native element with a graceful serpentine motion. 



At the base of each paddle is situated a smaller 

 one, consisting of a fleshy pedicle shielding a fan-like 

 bunch of hairs, each of which tapers to a sharp point. 

 Combined, these hairs or spines form a powerful 

 defensive weapon, which can be extended or re- 

 tracted at will ; and it also serves as an oar, or 

 propeller. 



As a noteworthy instance of tenacity of life in the 

 lower animals, it may be well to mention here that I 

 have on various occasions, by aid of the microscope, 

 watched for several minutes the bunch of spines, 

 above alluded to, thrust out and retracted in a single 

 segment cut from the body of the Nereis ; and only 

 as the object became devoid of moisture did its beauti- 

 ful mechanism cease to play. 



The specimen now before me is comparatively 

 small, being only twelve inches in length, yet its body 

 contains nearly one thousand lateral appendages, con- 



