POLYZOA 193 
the lophophore ; it is a contractile body, being, with the tentacles, 
protruded from the zocecium or drawn within it at will) In 
some species the orifices of the zocecia 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 zocecium. 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 zocecia 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 are constantly taking place. 
The colonies originally start from larvee 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 zowcium. 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 GYMNOLEMATA 
SUBORDER CYCLOSTOMATA 
ERECT OR CREEPING POLYZOA 
The zocecia are more or less cylindrical; the upper ends are 
sometimes completely free and sometimes closely adherent. The 
13 
