ANELLATA. 131 



are confluent, and in almost all there exists at the base of each a 

 long soft cylindrical appendage called the "cirrus (c)." The bristles 

 in the setigerous Anellides 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 against which they strike. 



The nervous system of the Anellides presents a marked advance 

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

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

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

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

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

 ganglion. 



Most of the Anellides are provided with ocelli, and in many of 

 them the head supports soft cylindrical tentacules called "antennae :" 

 they are obviously organs of touch, but diiFer from the antennae 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 endowment of the 

 higher articulate classes, we have again an interesting illustration of 

 the principle of vegetative repetition ; for every setigerous tubercle 

 in the Anellides with cephalic antennae, has a similar organ of sen- 

 sation : the distinction is merely local and nominal ; the feelers on 

 the first segment being called "antennae;" those on the other seg- 

 ments " cirri." 



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

 extremity of the body in the acephalous Anellides ; in some species 

 it is provided with a protractile proboscis, and with lateral jaws in 

 the form of curved dentated horny plates ; and the alimentary canal is 

 generally straight, and in some species simple ; in others, pro voided 

 with a greater or less number of lateral caeeums. The anus is 

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

 degree of redness of the circulating fluid varies considerably ; in 

 some species it is very pale : in one or two it 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 pul- 

 sations ; and in some species the circulation is further aided by 

 contractile sinuses, called hearts. 



All Anellides 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, and vary considerably in form and position. 



Such are the general anatomical characters of the class Anellata^ 



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