STRUCTURE OF ANNELIDANS. 80 



LESSON VIII. 



CLASS OP ANNE'LIDA. Organization Division Earth- 

 worms. 



FAMILY OF SUCTO'RIA. Leech. 

 ORDER OF DORSIBRANCHIA'TA. Eunice. 

 ORDER OF TUBICOLA. Sabella. 



CLASS OF ANNELIDA.* 



The lowest class of articulated animals comprehends an exten- 

 sive series of creatures generally grouped together under the com- 

 mon name of worms. 



1. The class of anne'lidans is composed of red-blooded worms, 

 and is easily distinguished from the rest of the Branch of articu- 

 lated animals by the absence of articulated extremities. 



2. The body of these animals is considerably elongated, and 

 generally slender (figs. 76 and 79) ; it is composed of a succes- 

 sion of numerous rings, the first of which, although it differs but 

 little from the others, may be called the head ; it contains the 

 mouth, which is sometimes armed with a formidable apparatus 

 of jaws. The skin has little consistence, and the rings formed 

 by it are never horny nor stony. Many anne'lidans are entirely 

 without legs, an example of which is seen in the leech (fig. 76) ; 

 and when these organs do exist, they are never formed of solid 

 pieces, articulated end to end, as in insects, crusta'ceans, and 

 arach'nidans ; they are merely fleshy tubercles, armed with stiff* 

 setae or movable bristles, and are arranged in pairs on each side 

 of the body, and are commonly found on each ring. The figure 

 (73) on the next page, represents a transverse section of an anne'li- 

 dan, and conveys an idea of the character of the extremities of 

 these animals; d, is the dorsal arch of the ring; v, the ventral 

 arch ; rv, an extremity of the ventral arch ; rd, an extremity 

 of the dorsal arch; 5, setse or bristles, surrounding the append- 

 age, called cirrus (e). The Eunice (Jig. 79), a marine worm 

 often found on oysters, is an example of an animal having extre- 

 mities of this kind. 



3. The nervous system consists of a long series of minute 



* From the Latin, annulus, a little ring. 



1. How are anne'lidans distinguished from other articulated animals ? 



2. How are anne'lidans characterized ? 



3. What is the character of their nervous system ? 



