ANNELIDA. 



271 



either the right or the left side, and no end that 

 could be properly said to be the front, or the 

 back, in Articulated, as well as in Vertebrated 

 animals, all these distinctions are clearly marked 

 and easily defined. 



In all the Annelida the firmest parts of the 

 body, or those which give mechanical support to 

 the rest, are external, and may be regarded either 

 as appendages to the integuments, or as modifi- 

 cations of the integuments themselves. They 

 consist of a frame- work, composed of a series of 

 horny bands or rings : their assemblage having 

 more or less of a lengthened cylindric shape, 

 and constituting a kind of external skeleton, 

 which encloses all the other organs. This is 

 exemplified in the earth-worm ; in the Pont- 

 obdella (Fig. 128), which is a species of leech ; 

 and in the Nereis (Fig. 129). These rings 



129 



128 



give rise to the division of the body into as 

 many different segments. In some cases, how- 

 ever, we find all these rings compressed into 



