Geneea of the N'ortii American Palaeozoic Bryozoa. 431 



larium with a captive worm, longer than the polyp cell ; of 

 course the avicularia can not convey food to the mouth, and the 

 animal is incapable of swallowing any but the most minute 

 particles. Any capture by an avicularium is clearly accidental, 

 caused by the mandible shutting as the particle floated beneath 

 it, and the constant flapping of the mandible would tend to 

 create a current away from it. According to other authors its 

 functions are purely defensive. "They may either arrest or 

 scare away unwelcome visitors. Their vigorous movement and 

 the snapping of their formidable jaws may have a wholesome or 

 deterrent effect on loafing annelids or other vagrants, while the 

 occasional capture of one of them may help still further to pro- 

 tect the colony from dangerous intrusion.'' (Hincks.) But the 

 avicularia occur only on the Cheilostomata, and as other forms 

 are without them, it shows that they are unnecessary as weapons 

 of defense. 



Figs. 10, 11 Two views of a pedunculate avicularia cf Bicellaria pectogemma, showing occlusor 

 and retractor muscles (after Busk). 



The vibracula consist of long slender setae or bristles thickened 

 near the base (figs. ^"' and 8, v). They are divided by Busk into 



two kinds, as follows 



