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ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1920. 



change of temperature and the danger of drying up is ever present. 

 Their special characteristic is the habit of dying down in the winter 

 with the formation of the so-called statoblasts, hard-shelled repro- 

 ductive bodies consisting of internal buds protected by a chitinous 

 shell capable of resisting unfavorable conditions for a period and 

 then forming new zooids. Sexual reproduction also occurs as in 

 other bryozoa. The Phylactolaemata have a body structure some- 

 what similar to the Ctenostomata, the first order of the Gymnolae- 

 mata, which also show a tendency to live in fresh water, and from 

 which these exclusively fresh-water bryozoa are believed to have 

 been derived. They are often quite common in a zone of water about 

 2 feet below the surface, where their colonies are found attached to 

 water plants or stones. A few forms are found in running water 

 but most of them occur in still water. 



Fig. 4. — Cristatclla muccdO Cuvier, a fresh-water bryozoan from England, X24, a 

 typical member of the Phylactolaemata, showing slug-shaped body and the 

 horseshoe-shaped lophophore (1) ; a statoblast (2) of the same species, X28. 



The zoarium may consist of gelatinous masses of varying size, of 

 aggregations of parallel tubes or of single branching tubes, in all of 

 which cases the body cavities of the zooids are continuous with each 

 other. The body cavity in the Phylactolaemata is thus a continuous 

 space, while in the Gymnolaemata each zooid has its own body wall. 

 As in the Entoprocta, the body wall is uncalcified, and fossil forms 

 are not to be expected. Protrusion of the polypide is effected by the 

 contraction of the muscular body wall compressing the fluid of the 

 body cavity. The tentacles sometimes interlace to form a sort of cage 

 in which infusoria used for food are imprisoned. 



Cristatella (text fig. 4) , a typical member of the superorder, consists 

 of a slug-shaped gelatinous mass, sometimes eight inches long but 

 only one-half inch wide with a flattened sole on which it has the 

 power of crawling. The protruding polypides form a delicate fringe 



