ANNULATA. 227 



and in almost all there exists at the base of each a long soft cylin- 

 drical appendage (c). The bristles in the setigerous anellids are 

 their chief organs of locomotion, and at the same time their weapons 

 of attack and defence. They are generally sharp, or barbed, and 

 hard enough to readily penetrate the soft bodies which they strike. 



The nervous system of the anellids presents a marked advance 

 beyond its condition in the white-blooded parasitic worms ; it consists 

 of a double median central chord or chain of small ganglions, ex- 

 tending from one end of the body to the other ; the two chords diverge 

 anteriorly to allow the passage of the oisophagus, and again unite 

 above that tube to form a distinct, though small, bilobed cephalic 

 ganglion or "brain." 



Most of the anellids are provided with pigment- or eye-specks, 

 '* ocelli ; " and in many of them the head supports soft cylindrical 

 tentacles, which are obviously organs of touch, but differ from the 

 antennjB of insects in the absence of joints. In the first appearance 

 of these not yet well understood organs of sensation, which form so 

 remarkable and conspicuous a character and so important an endow- 

 ment of the higher articulate classes, we have again an interesting 

 illustration of the principle of vegetative repetition ; for every seti- 

 gerous tubercle in the anellids with cephalic feelers has a similar 

 organ of sensation : the distinction is merely local and nominal ; the 

 feelers on the first segment being called "antenna;" those on the 

 other segments " cirri." 



The mouth is at the lower surface of the head, or at the anterior 

 extremity of the body in the acephalous anellids ; in some species it 

 is provided with a protractile proboscis and with lateral jaws in the 

 form of curved dentated horny plates: the alimentary canal is gene- 

 rally straight ; in some species simple, in others sacculated, or pro- 

 vided with a greater or less number of lateral ceecums : the anus is 

 situated above, or at, the posterior extremity of the body. 



The degree of redness of the circulating fluid varies considerably; in 

 some species it is very pale; in one or two the fluid even presents a 

 greenish hue : it circulates in a closed and very complicated system 

 of vessels, of which the chief dorsal one is distinguished by its 

 undulatory pulsations ; and in some species the circulation is further 

 aided by contractile sinuses, called hearts. 



All anellids have organs of respiration, adapted in a few species 

 for extracting oxygen directly from the atmosphere, and in the rest 

 of the class through the medium of water : in these the gills are 

 usually external, vary considerably in form and position, and form 

 the only part of the external surface which retains the ciliated 

 epithelium. 



Q 2 



