ON ANNELIDA. 
55 
cirri, of nipple, and of setae, which we must proceed now to 
define successively. 
The gill (for we have only occasion to consider a single side, 
as the animal is symmetrical), is always situated at the upper 
root of the appendage, whether simple or bipartite. This gill, 
which exhibits a character common to every organ of respira- 
tion, namely, that of being extremely vascular, with a very 
slender dermoid envelope, varies sufficiently in its form, a§ it 
may be either simply bifid or trifid, as in the nereides, or con- 
siderably ramified into arbuscula, as in the amphinoma ; or, 
in fine, multifid, and longitudinally pinnate, as in the ser- 
pulacese. 
Their position allows us to distinguish them into many 
kinds : in the regular state, they constantly occupy the upper 
root of a variable number of appendages, and they are dorsal, 
as in the amphinoma;, and the larger species of nereis. In 
that case, a number of them may be attached to all the rings 
without interruption. At other times they are anterior, and 
confined but to some rings. 
The cirri, which we shall see might be termed tentacula , 
or tentacular cirri , on the cephalic , or post -cephalic rings, 
are species of filaments not vascular, in length and even in 
form extremely variable, which may be situated either at the 
upper part of the appendage, immediately under the gill, when 
that exists, and which sometimes seem even to take the place 
of it ( cirrus superior J, or at the lower or ventral portion of 
the appendage ( cirrus inferior or ventralis.) The form and 
proportional dimensions of the latter vary considerably, but in 
general it is smaller than the upper. 
Sometimes we find at the root of the fasciculi of seta), 
behind or before, dermoid prolongations of the nature 
of cirri, but which, shorter and broader, no longer deserve 
this name, yet are still worthy of notice. They may be 
