AYICULARIA AND VIBRACULA. 



393 



shuts down on the retracted pol3^pide. This opercidum is 

 placed on the opposite side of the polj^pide to that on which 

 the nervous ganglion 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, w^ith 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. 



Fio. ^\^.— 'Bvnnla amcularia.—A. Part of tha polyznarium viewed from the neural 

 Pide, showin? the tentacles of a polypide protruded from its cell (A); the intestine 

 (n and the stomach and eullet (/) ; q, retractor muscles ; d, d. avicuhina. 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 (rf), viewed from the hsemal or dorsal 

 side. 



The dilated bases of the vibracula contain muscles by the 

 contraction of w^hich 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 



