AVICULARIA AND VIBRACULA. 



393 



shuts down on the retracted poljpide. This operculum is 

 placed on the opposite side of the polypide to that on which 

 the nervous gauglion is situated. 



In many genera, the cells are provided with flagelliform 

 appendages— the vibracula (Fig. 114). These are usually 

 articulated with short dilated processes of the ectocyst, and 

 execute constant lashing movements. In others bodies 

 shaped like birds' heads, with a movable mandible, and either 

 seated upon slender and flexible peduncles or sessile, snap 

 incessantly. Sometimes these last, which are termed ^avicu- 

 laria (Fig. 115), are present along with vibracula. 



E^G. 115. — Bvqula avicularia.—A. Part of the polyzoarium viewed from the nenral 

 side, showing the tentacles of a polypide protruded from its cell (A); the intestine 

 (D and the stomach and gullet (/) ; <7. retractor muscles ; d. d, avicularia. One of 

 these is holding a minute worm which it has seized. In front of this is seen an 

 ovicell. 



B. A retracted polypide withan avicularium (c?), viewed from the haemal or dorsal 

 side. 



The dilated bases of the vibracula contain muscles by the 

 contraction of which the flagelliform appendage is moved. 

 In the avicularia, a large adductor muscle, which takes its 

 origin from the greater part of the inner surface of the 



