350 



ZOOLOGY 



SECT. 



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

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

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

 cavities of the neighbouring zooecia 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 

 described in the case of Bugula, covered only with a thin and 



FIG. 281. Cristatella xnucedo. Entire colony. (After Allman.) 



flexible cuticle, and forms an introvert capable of being retracted 

 into the interior of the zooecium. At the free end of the introvert 

 is the mouth surrounded by a lophophore bearing tentacles. The 

 tentacles are always simple, filiform, and hollow, each containing a 

 narrow diverticulum of the circular canal or anterior compartment 

 of the ccelome. They are beset with vibratile cilia by means 

 of which currents are created subserving alimentation and 

 respiration. They are also highly sensitive, and are capable 

 of being bent about in various directions by the contraction 

 of muscular fibres in their walls, so that they can be 



