WORMS, MOSS-POLYPS, SPONGES, ETC. 1183 
vantage certain structures that are not present in 
all the members of the class. These are the ‘ bird- 
head processes’ (avicularia), little snapping bodies 
of the general shape of a bird’s head, which are 
found attached to the cells, but of whose real 
functions we as yet know very little. That they 
seize by a sort of instinctive movement little aquatic 
objects has been long since demonstrated, but the 
wherefore of this action still remains to be as- 
certained, since the particles thus caught can- 
not readily be turned to account by the animal. 
Other forms have in place of these avicularia long 
lashes or whips (vibracula), which are in almost 
constant motion, and may, at least in part, serve to 
keep the colony clean from adhering particles. The 
movement of both of these or- 
gans can be easily followed in the 
field of the microscope. 
A beautiful rosette-formed poly- 
zoan, Crisia eburnea, whose attach- 
ment is the frond of the sea-weed, 
is not rarely found scattered over 
the sands; it is easily distinguished, 
apart from its manner of growth, 
by the calcareous or limy character of its ivory- 
white habitations. Another white form (Pedicellina 
Americana), with minute club-shaped individuals, 
weaves a delicate tracery around the branches or 
stems of other polyp colonies, hydroids, ete. 
CRISIA. 
h 10* 
