ANNELIDA. 



243 



lower surface, and there was no side which could 

 be exclusively denominated either the right or the 

 left side, and no end which 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 modifications 

 of the integuments themselves. They consist of a 

 framework, 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 Lumbriciis, or 

 earth-worm; in the Pontohdella (Fig. 128), which 

 is a species of leech ; and in the Nereis (Fig. 129). 



These rings give rise to the division of the body 

 into as many different segments. In some cases, 

 however, we find all these rings compressed into 

 the form of a flat oval disk. This is the case in 

 the Erpobdella, of which Fig. 130 is an enlarged 

 representation. 



