POLYZOA 193 



the lophophore; it is a contractile body, being, with the tentacles, 

 protruded from the zooecium or drawn within it at will. In 

 some species the orifices of the zooacia are surrounded with spines ; 

 others have lids, called opercula, which shut down when the poly- 

 pides are retracted. A singular modification of the zocecia is 

 seen in the genus Bugula, where peculiar shapes like birds' heads, 

 called avicularia, occur. In these singular appendages the upper 

 beak is hooked, while the lower one, or mandible, is the operculum 

 of the zooecium. These constantly open and shut by means of 

 muscles, and although their function is not certainly known, it is 

 thought that they capture prey. Other modifications are whip- 

 like processes, or flagellate filaments, called vibracula, which con- 

 stantly beat the water. In many of the zoo3cia of the colony 

 there are dark-colored masses, called brown bodies. These are 

 formed by the degeneration of the polypides, which at certain 

 periods break down, their tentacles and alimentary system 

 ceasing to act and becoming brown masses. The body- wall then 

 puts out a bud internally and forms a new polypide, which ab- 

 sorbs, or passes out, the brown body. The term of life of a sin- 

 gle polypide is not known, but in every colony many such trans- 

 formations arc constantly taking place. 



The colonies originally start from larvae produced in the ovi* 

 cells of the zoaria. The ovicells are sometimes very large and 

 pear-shaped, occurring at intervals on the stems ; or there may 

 be one at the top of each zoo3cium. They look like pearls. A 

 larva, when liberated, swims about for a time, then attaches itself 

 to a fixed object, and increases, by budding, into some one of the 

 various forms which the colonies take. 



SUBCLASS ECTOPROCTA 



ORDER GYMNOL2EMATA 



SUBORDER CYCLOSTOMATA 



ERECT OR CREEPING POLYZOA 



The zooecia are more or less cylindrical ; the upper ends are 

 sometimes completely free and sometimes closely adherent. The 



