POLYZOA. 
55 
the edges of little oblong boxes or cells termed zocecia,* arranged in 
longitudinal parallel rows and forming a double layer back to back. 
The cells are broad and rounded above, narrow and truncate below, 
and each is roofed in by a transparent membrane with a semicircular 
lid or operculum situated near the upper end ; four short stout 
spines spring from the margin in this neighbourhood. When the 
surface of a living frond is examined in sea-water, here and there 
a bundle of tentacles may be observed pushing up a lid, slowly 
emerging and expanding into a bell-shaped coronet ; on the least 
alarm the tentacles are rapidly withdrawn into the cell and the lid 
Fig. 2. Fig. 3. 
Fig. 4. 
Fig. 5. 
Figs. 2, 3, diagrams representing polypide in cell. Fig. 2, tentacle-sheath pro- 
truded. Fig. 3, ditto, retracted ; a, tentacles ; b, tentacle-sheath ; c, mouth : 
d, gullet ; e, stomach ; /, vent ; g, retractor muscle ; h, funiculus ; I, ovary ; 
h, testis ; I, lid or operculum ; nerve ganglion is between mouth and vent. 
Fig. 4, polypide extracted from cell ; d, pharynx ; e, stomach ; /, anus (after 
Van Beneden). Fig. 5, section (partly diagrammatic) of frond of Flustra, 
showing cells back to back. 
shut. The flexible protrusible region of the cell is termed the 
tentacle sheath. The relation of the cell to the tentacle sheath 
(Figs. 2, 3) may be roughly compared to a glove finger, stiff below, 
but flexible at the end, and surmounted by a crown of bristles ; on 
pulling down the glove-finger tip, the tentacles will also be drawn 
in, and will lie in a sheath formed by the invaginated portion of 
glove finger. The lid which closes over the tentacle sheath is only 
found in the Sub-order Chilostomata to which Flustra belongs. The 
area of the tentacle sheath whence the tentacles arise is termed the 
lophophore.t 
* Zoon, animal ; oikos, house. 
t Lophos, plume ; pherein, to bear. 
