sibly it is poisoned by nitrogenous wastes, with no 

 excretory system to discharge them to the outside 

 world. This seems improbable, since oxygen is ex- 

 changed for carbon dioxide with no respiratory sys- 

 tem, merely by absorption and release through thin 

 areas of the body wall, including the crown of tenta- 

 cles. Perhaps the zooid dies by rupture in freeing its 

 sexually produced oflFspring. The timing would cor- 

 respond well. And a habit of this kind would not be 

 without precedent, whether in the clamworms or the 

 sea lamprey or Pacific salmon. 



Each active zooid is a sort of jack-in-a-box, often 

 concealed by a little trap door. Contraction of certain 

 body muscles causes an increase in the hydraulic 

 pressure inside the body cavity. This pressure is re- 

 lieved when the crown of tentacles moves through the 

 doorway into the surrounding water. Additional mus- 

 cles may even widen the opening, as though to speed 

 the tentacle crown on its way. Yet the first disturb- 

 ance is enough to cause still other muscles to drag 

 the everted crown back into the safety of the cham- 

 ber. If it has a lid, the door snaps shut. 



In order to use this hydraulic method in extending 

 the crown of tentacles, each zooid must fill its cham- 

 ber. It must also retain space to accommodate the 

 tentacle crown whenever this feeding organ is en- 

 dangered. These conflicting requirements leave no 

 space for growth. Nor does the organization of the 

 colony provide for the general expansion of individu- 

 als. The colony grows by addition of new chambers 

 and new zooids. 



The permanence of a bryozoan colony is improved 

 if the chambers have thickened walls. The zooids are 

 also better supported, and can reach more success- 

 fully into the water for particles of food. Yet allow- 

 ance must still be made for the out-and-in movements 

 of the tentacle crown. Several solutions to this prob- 

 lem are possible, and different bryozoans have be- 

 come adapted in one way or another. 



Some moss animals with armored chambers leave 

 one wall — usually an end one, overhung by protective 

 spines — thin and membranous, flexible enough to be 

 pulled in while extending the tentacles, and to bulge 

 out again when the feeding organ is retracted. Other 

 bryozoans possess a second, special "compensation 

 cavity" within the chamber, with its own small open- 

 ing to the outside world. Muscles that dilate the com- 

 pensation cavity squeeze the zooid and cause the ten- 

 tacle crown to pop out. Retraction of the crown forces 

 water from the compensation cavity, giving the zooid 

 space within its box. 



A third method seems still simpler: the contraction 

 of muscles dilating the opening of the chamber com- 

 presses the fluid in the zooid's body cavity and ejects 

 the tentacle crown. Retraction is at the expense of 

 space in the chamber opening through which the feed- 

 ing organ is withdrawn. 



So many ditTerent forms of life compete for space 

 on rocks and shells, pilings and boat bottoms, that 

 bryozoans are ever in danger of being overgrown. In 

 many places, suffocation under inedible sediments is 

 another hazard for all attached animals. Seemingly to 

 deal with this dual jeopardy, most colonies of bryo- 

 zoans in the larger of the two marine orders support 

 non-feeding zooids serving to police the immediate 

 world of the feeding individuals. That this should also 

 be the order in which hinged doors is usual is no co- 

 incidence. The defending zooids are merely ones 

 whose trap doors are jiioditied as tools. 



One type of policing zooid is the avicularium, in 

 which the movable door has become the jaw of a 

 snapping individual resembling a beaked bird's head, 

 and named from the Latin civicula, a little bird. The 

 other type has the door modified into a long, whip- 

 like organ that can be swept over the surface of the 

 colony. Avicularia seize and hold small invaders of 



A large colony of a fresh-water bryozoan. Pectinatella 

 magnifica, foinid growing on submerged wood in 

 quiet water. This massive species is common east of 

 the Mississippi valley. (Pennsylvania. Ralph Buchs- 

 baimi ) 



