STRUCTURE AND FUNCTIONS OF V1BRACULA. 441 



have enjoyed in watching the ludicrous movements of 

 these anomalous productions. 



With regard to the development of the " Avicu- 

 laria," little has been made out. At first, they make 

 their appearance as convex or hemispherical knobs, 

 surmounting a simple cylindrical stem, and as growth 

 advances progressively, the bird's-head form is gradu- 

 ally attained. 



In some species of Polyzoa, a moveable appendage 

 of a totally different description, named the " vibra- 

 culum" affords an interesting subject for microsco- 

 pical observation. This consists of a hollow filament, 

 situated at the upper and outer angle of each cell, 

 filled with a fibrous contractile substance, whereby 

 it is enabled to effect movements of a very remark- 

 able character. These movements occur at irregular, 

 occasionally very short, intervals. First, the filament 

 sweeps downwards over all the posterior surface of 

 the polypidom within its reach, and then returning on 

 its former track, descends in the opposite direction. 

 The use of this singular apparatus is at present con- 

 jectural, for its movements are quite independent of 

 the polyp, and continue for days after its death. It 

 has been suggested, that it may be useful, by serving 

 to rid the Polyzoon of intruding vagrants, and to 

 cleanse away accidental defilement by sweeping across 

 the orifice of the cell. 



Such are the principal and most instructive fea- 

 tures of the Polyzoa met with upon our coasts ; but, 

 because we have selected only a few of the more 

 ordinary species for the illustration of their general 

 habits, the reader must by no means infer that the 



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