pinches itself into two or more pieces, and each goes 

 off on its own, elongating as new zooids are added. 

 By late autumn, a lily pad floating on a pond particu- 

 larly rich in food for bryozoans may wear a whole 

 mesh of Cristatella, where separate colonies have 

 fused together. 



Marine Bryozoans 



{Class Gymnolaemata) 



The zooids of marine moss animals have no little 

 flap of body wall projecting over the mouth, and 

 hence the gullet is exposed, as is indicated by the 

 name of the class (from Greek gyiniio, naked, and 

 laiinos, the gullet). In these bryozoans the tentacle 

 crown is circular. The body wall contains no muscle 

 layer, and each zooid has a separate body cavity 

 (coelom). 



A great many marine moss animals are cosmopoli- 

 tan, apparently having been carried throughout the 

 world on floating seaweeds, drifting wood, and the 

 bottoms of ships. Consequently a remarkable variety 

 can be found on almost any rock covered by water 

 at low tide, on any wharf piling that has stood for a 

 season, or among the fouling organisms on a boat 

 bottom. 



Each of the three methods by which moss animals 

 provide for hydraulic extrusion of the tentacle crown 

 is characteristic of an order. Those retaining at least 

 one wall of the chamber as a thin, flexible membrane 

 may thicken the other walls but not impregnate them 

 with lime. These bryozoans wear around the extruded 

 feeding organ a pleated collar with stiffening rods, 

 suggesting a circular comb. They use this device to 

 close the opening of the chamber after the tentacles 

 have been withdrawn, and are the "comb-mouths" of 

 order Ctenostomata. One of the strangest of them 

 is Victorella pavida, whose long, slender, vaselike 

 chambers arise from a branched, vinelike growth at- 

 tached to underwater objects. It was discovered first 

 on docks in the Thames River at London, and not 

 only tolerates brackish water to a degree unusual 

 among bryozoans but seemingly lives also in the fresh 

 waters of Lake Tanganyika in Africa, on stones and 

 shells and in cavities of fresh-water sponges there. 



Members of several ctenostome families specialize 

 in dissolving their way into the limy shells of conchs 



and other heavy marine mollusks, replacing the mate- 

 rial removed by thin-walled chambers of their own. 



Bryozoans whose chamber walls are all calcified 

 have circular openings, and exchange space in the 

 narrow vestibule opening to the sea for space in 

 the body cavity when the tentacle-bearing crown is 

 pushed out or pulled in. These are the "narrow- 

 mouths" of order Stenostomata. None of them pos- 

 sesses avicularia or the whip-wielding vibracula, but 

 reproduction may include a technique found nowhere 

 else among bryozoans: they produce a number of em- 

 bryos from each fertilized egg — like identical quintu- 

 plets, except still more numerous. The largest genus 

 (TiibuUpora) includes many kinds forming prone or 

 erect colonies, often expanded into fanjike clusters 

 from which the reproductive zooids project as clearly 

 specialized members of the population. 



The remaining marine bryozoans either retain one 

 membranous wall in the calcified chamber or produce 

 a compensation cavity. The opening of the chamber 

 is usually protected by a movable door, as the "lip" 

 referred to in the name of the order Cheilostomata, 

 the "lip-mouths." This order is the only one in which 

 some zooids are modified into avicularia or vibracula, 

 serving to keep the colony immaculate and unin- 

 vaded. Most marine bryozoans are cheilostomes. 



One of the most striking and largest of the cheilo- 

 stomes is the barely calcified Biigula tiirriia, colonies 

 of which are treelike, with branches each a spiral tuft 

 of flat, fan-shaped groups of branchlets. Double rows 

 of zooids on each branchlet have the openings facing 

 in a single direction, the surface over which the avicu- 

 laria patrol. These guardians swing on slender, flexi- 

 ble necks, back and forth with beaks wide open. 



Fully developed Biigula colonies may protrude 

 from wharf pilings and sea walls to a distance of 12 

 inches, the bright yellow or orange tentacles con- 

 trasting with the dark water anywhere from Maine 

 to Brazil. Other species of Bugiiki are widely distrib- 

 uted in the northern and eastern Atlantic (Plate 34). 



A very diff'erent type of cheilostome is the sea mat 

 Fhistra joliacea. so abundant on the shores of western 

 Europe. Often it is mistaken for a seaweed, for it 

 forms erect, leafy colonies. Each surface of the broad, 

 bladelike branches is densely fitted with zooid-con- 

 taining chambers, each with two little horns. Among 

 the zooids are scattered, smaller, rounded avicularia 

 with broad lips suggesting the distorted mouths of 

 Ubangi women in the Belgian Congo. 



1153 



