342 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



and close with a snapping motion, which has given rise to the 

 probably erroneous idea that they are organs of defense. These 

 two organs are mentioned in more detail in the consideration of the 

 cheilostomatous bryozoa. Both the avicularia and vibracula are 

 incapable of preservation in the fossil state, but their former pres- 

 ence is indicated by the porelike excavations in which they lodged. 

 The extended polypide is withdrawn into the zooecium by the 

 contraction of retractor muscles attached to the tentacular crown. 

 In the bryozoa with flexible zooecia the contraction of the body 

 walls by parietal muscles produces protrusion of the polypide, but 



orifice 



diaphragm 



tentacular sheath 

 tentacles 



parietal muscles 



intertentacular organ' 

 mouth 



etractor muscles 



Fig. 1. — Anatomy of the polypide. Diagram showing the anatomy of 

 a single zooecium of Alcyonidium albidum Alder, highly magnified, 

 with its polypide retracted. (After Prouho.) 



in the rigid calcareous zoaria the means for protrusion are more 

 complicated as explained under the Cheilostomata. 



The zooecium, the protective covering of the polypide, varies so 

 much in structure that its description is reserved for the discussion 

 of each order. 



CLASSIFICATION. 



The first serious attempt at a classification was made by D'Or- 

 bigny, 1 whose wide acquaintance with recent and fossil bryozoa has 

 perhaps been equaled by no subsequent writer. But the system he 

 devised was so largely artificial and burdened with so perplexing a 



1 1850-1852. D'Orbigny. Paleontologie francaise, Terrain Cretac6, Vol. V. 



