342 



ZOOLOGY 



SECT. 



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 

 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. 286) in which it performs creeping 

 movements, in some other (American) forms of Phylactoleemata 

 (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. 



stato 



F , 



KIQ. _!S5. Plumatella. Portion of a colony, magnified, fnnlc. funiculus ; tjung. ganglion; 

 int. intestine ; mo. mouth : a?, oesophagus ; rtpr. gonad ; retr. retractor muscle ; si. stomach : 

 stato. statoblasts. (After Allman.) 



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

 circular, or crescentic apertures, which in the Phylactolsemata and 

 the Cyclostomata among the Gymnola3mata are devoid of any special 

 closing apparatus ; while in the Cheilostomata there is a movable 

 lid or operculum closed by a pair of occlusor muscles when the 

 introvert is retracted ; and in the Clenostomata there is a series of 

 lobes or teeth which close in together over the opening. The 

 cavities of the neighbouring zocecia are in some forms completely 

 cut off from one another by a continuation of the chitinous or 

 calcareous exoskeleton ; in others there is free communication ; in 

 others, again, there is communication through a number of minute 

 perforations. 



The oral (anterior) part of the body of each zooid is, as already 



