ANNELIDANS. 343 



trading the rings, they bring up the posterior 

 portion of their body, and then fixing that part, 

 extend the anterior rings, and so proceed suc- 

 cessively with a kind of undulating motion. 



3. We are next to notice a tribe of Annelidans, 

 many of which, in one respect, make some ap- 

 proach to the Testaceous Molluscans. Though 

 truly annulated and furnished with a kind of false 

 legs, they are defended by a shell resembling 

 in its substance, that of the class just alluded 

 to, but often by its irregular convolutions prov- 

 ing that it belongs to an Annelidan and not to 

 a Molluscan ; some indeed approach to the spiral 

 convolutions of a Trachelipod shell ; others form 

 a membranous sac, and cover it with agglutinated 

 particles of sand, as the common Sabella ; others 

 again, likewise inhabit a tube, but they fix it in 

 the rocks. The testaceous animals of this class, 

 particularly the worm-shells, 1 inhabit a tortuous 

 tube which they form, probably with more ease 

 and celerity than the Molluscans form their 

 shells for they appear almost to do this as they 

 move, since the shape of the shell imitates the 

 sinuous windings of a worm, and that of the 

 Serpula adheres to the substances on which it 

 is formed. We see it often upon the shells of 

 bivalves, to which it adheres by the lower sur- 



1 Serpulidce. 



