Geneea of the JSTorth American Palaeozoic Bryozoa. 459 



" We thus clearly see that the set of radiating muscles is a 

 compensation for the absence of setae. 



" On examining an animal of Bowehbankia in action it is evi- 

 dent that the use of the setae is as I have pointed out. The 

 circle of setae is then seen to encompass the lower part of the 

 extended polyp, and when the tentacular disc moves from side to 

 side the neck always bends from the top of the setae in a decided 

 angle and does not gradually arch away from the lips of the cell, 

 as might be expected if this contrivance was for the purpose of 

 giving flexibility. The delicate membrane uniting the setae is 

 strengthened with numerous minute transverse fibers forming 

 the whole into a powerful sphincter, thus giving great firmness 

 to the part. 



" By this arrangement BowERBA.»KrA is enabled to raise the ten- 

 tacular disc far above the polype cell and yet to remain as per- 

 fectly under the control of the muscles as is the tentacular disc 

 of Plumatella and Fkedericella, in both of which it is confined 

 close to the surface of the cell." 



Fig. 25. Bowerbankia densa, showing the manner of extrusion of the setae and tentacles; in fig. a, 

 they are entirely retracted; in fig. d they are completely extended, a, Setae; d, Muscles (after 

 Farre) . 



Pig. 25 {a, 5, c and d) shows the position of the setae when the 

 animal is contracted and during the process of eversion. 



