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, as 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 amphinomse, 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-ceplialic 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 apjDcndage, immediately under the gill, when 

 that exists, and which sometimes seem even to take the place 

 of it (cirrus superior), or at the lower or ventral portion of 

 the appendage (cirrus inferior oxventralis.) 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 vvhich, shorter and broader, no longer deserve 

 this name, yet are still worthy of notice. They may be 



