Ixiv 



INTRODUCTION. 



Modifications of the Zocecial type. 

 The Avicularium and Vibraculmn. 



The structural type of which the zooecium is the most 

 familiar representative, exhibits a number of modifica- 

 tions amongst the marine Polyzoa; of these the most 

 remarkable are the avicularium and the vibraculum. 

 These curious appendages are confined to a single sub- 

 order, the Ckeilostomata, within the limits of which they 

 occur in great abundance and variety. The vibraculum 

 (probably a derivative from the avicularium) is rarely 

 met with as compared with the latter, which is present in 

 a large proportion of the Cheilostomatous genera*. 



The avicularium is best known in its most highly 

 specialized form as it occurs in the genera Bugula and 



Fig. xxviii. 



h. Beak. md. Mandible. C. Chamber, f. Peduncle, om. Occlusor 

 muscles. dm. Divaricator muscles. 



* Out of forty-five British genera of Ckeilostomata, avicularia occur in 

 thirty-one, vibracula only in four. In two genera both the appendages 

 are present together. 



