ANNELIDA. 



195 



While in the Star-fish, and Ecliinus, nothing; in point of si- 

 tuation was delinite, excepting tlie upper and the lower sur- 

 face, and there was no side which could be exclusively de- 

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

 could be properly said to be the front or the back, in Ar- 

 ticulated as well as in Vertebrated animals, all these distinc- 

 tions are clearly marked and easily defined. 



In all the Jlnnelida 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 frame-work, composed of a series of horny bands 

 or rings: their assemblage having more orless 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. 120.) These rings 



give rise to the division of the body into as many difierent 

 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 Erpohdella, of which Fig. 130 is an enlarged 

 representation. 



In general, the first of the segments into which the body 

 is divided, contains the principal organs of sense, and is suf- 

 ficiently distinct from those which follow to entitle it to the 

 appellation of the head; while the Icngtiicned prolongation 

 of the opposite extremity, when such a form is present, may 

 be denominated the tail. 



