54 SUPPLEMENT i 



ever, visible in any of the chetopoda, no more than in any 

 other of the division articulata. 



The rings or segments of the body of the chetopoda are con- 

 stantly provided with a pair of appendages, either consider- 

 ably complicated, or extremely simple. These appendages 

 are never, even when most complex, composed of more than 

 three parts : one proper for locomotion, another for sensation, 

 and a third for respiration ; and when most simple, have at all 

 events the first. As to their position on the rings, the appen- 

 dages generally occupy the extremities of the greatest trans- 

 verse diameter ; but it sometimes happens that they are situ- 

 ated lower, and more frequently higher, according to the 

 peculiar uses for which they are intended. Generally speak- 

 ing, they tend more to an upper, or dorsal position, as they 

 appertain more to the anterior rings of the body ; so that when 

 placed upon the rings of the head, they may with propriety be 

 termed tentacula. The exact reverse is the case when they 

 are situated behind, their tendency being more and more to 

 an under position, in proportion as the rings approach to that 

 segment which contains the anus. 



The extent of the lateral portions occupied by the appen- 

 dage must, and in fact does vary, according to the complica- 

 tion of the latter. In the Naides we find it reduced to a mere 

 point, while in the amphinomae, and some of the nereides, it 

 takes up more than a fourth of the circumfierence of the ring. 



When the extent of the insertion of the appendage is con- 

 siderable, it frequently happens that it is divided into two 

 parts, one superior and the other inferior to the lateral line. 

 To these M. Savigny gives the name of oars, a name deserving 

 of adoption, as in fact they do serve the animal for the purr 

 poses of swimming. Each oar seems composed of the same 

 parts, but arranged in an inverse way, the separation taking 

 place in the fasciculi of setae. But be this as it may, the ap- 

 pendage in the chetopoda may be composed of a gill, of 



