viii PHYLUM MOLLUSCOIDA 349 



colony varies in different families and genera in accordance with 

 differences in the shape of the constituent zooecia, and differences 

 in their mode of budding and consequent arrangement. The 

 zocecia are sometimes tubular, sometimes ovoid, sometimes poly- 

 hedral. In some cases the buds are so developed that the colony 

 assumes the form of a thin, flat expansion, which may be encrusting, 

 and consist of a single layer of zocecia in close contact with one 

 another or connected together by tubular processes ; or may be 

 erect, and with the zooecia either in one or two layers : sometimes 

 the lamellar colony thus formed may be fenestrated or divided into 

 lobes ; sometimes it is twisted into a spiral. In other cases the 

 colony, instead of being lamellar, has the form of an erect, shrub- 

 like structure, consisting of numerous cylindrical, many-sided, or 



I /" tf^'Xar^F' * L*risl4-; 



state 



y 



FIG. 280. Plumatella. Portion of a colony, magnified, funic. funiculus ; gang, ganglion; 

 int. intestine ; mo. mouth ; a 1 , oesophagus ; repr. gonad ; retr. retractor muscle ; st. stomach ; 

 stato. statoblasts. (After Allman.) 



strap-shaped branches arising from a common root. Sometimes 

 there is a creeping cylindrical stolon, simple or branched, having 

 the zooids arranged along it in a single or double row. The colony 

 is free only in Cristatella (Fig. 281) in which it performs creeping 

 movements, in some other (American) forms of Phylactola3inata 

 (in the younger stages of the colony), in one family of the 

 Cheilostomata the Selenariidce, (in which it moves along with 

 the aid of certain peculiar appendages the vibracula to be 

 described subsequently), and in one or two other cases. 



The zooecia open on the exterior by means of circular, semi- 

 circular, or crescentic apertures, which in the Phylactolsemata and 

 the Cyclostomata among the Gymnolsemata are devoid of any special 

 closing apparatus ; while in the Cheilostomata there is a movable 



