8 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 14 



of individuals. The individuals (usually less than a millimeter in length) 

 are provided with a calcified or chitinous outer wall (zooecium) into 

 which the lophophore and tentacles are retractile (in the Entoprocta the 

 polypide is naked and the tentacles are simply rolled inward). The great 

 majority of the species are marine, distributed from the polar regions to 

 the tropics and from the shore line down to great depths. They are abun- 

 dant as fossils from the Ordovician to Recent time. 



Classes of the Bryozoa 

 Polypide naked, stalked ; tentacles rolled inward, not withdrawn into 



the zooecium ; anal opening within the tentacle ring 



ENTOPROCTA 



Polypide enclosed in a chitinous or calcified wall (zooecium) ; tenta- 

 cles retractile ; anal opening outside of the tentacle ring, . . . 



ECTOPROCTA 



Orders of the Ectoprocta 

 Mostly marine, with a circular tentacle ring. . GYMNOLAEMATA 

 Fresh-water Ectoprocta, with a horse-shoe shaped tentacle ring. . . 



PHYLACTOLAEMATA 



Suborders of the Gymnolaemata 



1. Zooecium chitinous, its opening usually circular, closed by a puck- 



ering membrane CTENOSTOMATA 



2. Zooecium calcified, its opening circular, not closed by an operculum. 



CYCLOSTOMATA 



3. Zooecial walls usually well calcified, opening by a movable opercu- 



lar valve like a little trap-door. . . . CHEILOSTOMATA 



Suborder GhEILOSTOMATA Busk, 1852 

 The cheilostomes form the dominant group among the recent Bryozoa. 

 The zooecia are chitinized and usually calcified, often heavily so. In all 

 but a few cases there is a chitinized operculum which operates like a 

 trap-door to open and close the aperture through which the tentacles are 

 extruded and withdrawn. 



Hydrostatic apparatus. As the walls are rigid, compensation is neces- 

 sary for the changes in internal pressure when the tentacles are protruded 

 or withdrawn and this is accomplished by two methods: 1. there is a 

 membranous area on the front of the zooecium which rises and falls with 

 the changes in pressure (Anasca) ; 2. when the front is solidly bridged 

 over (Ascophora), there is an internal water sac (compensation sac or 

 compensatrix) which fills and empties through a special pore. 



