POLYZOA. 



55 



the edges of little oblong boxes or cells termed zooecia,* 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 

 sm-face 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 alarm the tentacles are rapidly withdrawn into the cell and the lid 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. Fi.o-. 3. 



Fig. 4. 



Fig. 5. 



Figs. 2, 3, diagrams representing polj'pide in cell. Fig. 2, tentacle-slieath pro- 

 truded. Fig. 3, ditto, retracted ; a, tentacles ; b, tentacle-sheath ; c, mouth : 

 d, gullet ; e, stomach ; /, vent ; g, retractor muscle ; h, funiculus ; ■/, ovary ; 

 k, testis ; I, lid or operculum ; nerve ganglion is between mouth and vent. 

 Fig. 4, polypide extracted from cell ; d, pharynx ; e, stomach ; /, vent (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 Ohilostomata 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. 



