54 
SUPPLEMENT 
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 tent ac ala . 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 amphinomse, and some of the nereides, it 
takes up more than a fourth of the circumference 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 pur- 
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 seta3. But be this as it may, the ap- 
pendage in the chetopoda may be composed of a gill, of 
