1 INTRODUCTION. 



habitation forms the body cavity or coelome; this is closed by "the 

 diaphragm," a muscular band around the zooid, which connects it 

 with the wall of the zooecium. When the zooid is withdrawn into 

 its cell, the muscular band closes the aperture ; while a horny plate 

 (the "operculum") formed on the side of the diaphragm opposite 

 the anus, affords additional protection. 



The zooid is held in its position by a series of cords (or 

 " funiculi") and muscles ; the largest of the latter is the " retractor 

 muscle," by which the zooid is withdrawn into the zooecium. 



The zocecium of Flustra foliacea consists of a chitinous case 

 shaped like a flat oblong box, arched at the oral end. The 

 wall consists of two layers, the chitinous and partly calcareous 

 "ectocyst" and the internal " endocyst." The walls are com- 

 plete except for some small pores (or " communication plates"), 

 by which a certain connection is established between adjoining 

 zocecia. 



This is the structure of a typical member of the colony of 

 Flustra, but two other types are present. In the first, one part 

 of the zooecium bulges out to form a shallow marsupium or 

 pouch in which the young are reared. Such marsupia are only 

 modified organs, or parts of normal zocecia; these are known as 

 ooecia. [In some other Bryozoa they are formed by the modifica- 

 tion of entire zooecia, when they are called " goncecia," or of 

 parts of the zoarium, when they are named " gonocysts."] Other 

 individual Bryozoites in the Flustra colony are modified in a 

 different way. The Bryozoite is dwarfed, and there is no zooid ; 

 the operculum is enlarged into a triangular plate, which is 

 hinged, and can be used as a beak, to hold anything that falls 

 within its grasp. Such a Bryozoite is an " avicularium " of the 

 simplest type. 



If we turn from Flustra to other Cheilostomata, we find 

 important zocecial differences. The avicularia become more 

 specialized, and instead of the marsupia being only ocecia, they 

 are gonoecia. But, nevertheless, the characters of the animal 

 of the normal Bryozoites remain very much the same throughout 

 the order. In fact, the differences between the zooids of the 

 Cheilostomata, and those of Ctenostomata and Cyclostomata, are 

 comparatively insignificant. Hence Flustra serves as a type for 

 the whole group of Gymnolaemata, which includes all the fossil 

 Bryozoa. 



