322 ZOOLOGY SECT, 



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 againyrfiere 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 

 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 coalome. They are beset with vibratile 



state 



FIG. 258. Plumatella. Portion of a colony magnified, gang, ganglion ; int. intestine ; mo. 

 mouth ; ce. oesophagus ; repr. reproductive gland ; retr. retractor muscle ; st. stomach ; 

 stato. statoblasts. (After Allman.) 



cilia by means of which currents are created subserving alimenta- 

 tion 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 are capable of being 

 used for prehension. In the PhylactolaBmata (Fig. 258) the 

 lophophore is horse-shoe-shaped, in the Gymnolaamata (Fig. 255) 

 circular : in the former, but not in the latter, there is a ciliated 

 lobe the epistome (Fig. 259 cp) which may have a sensory func- 

 tion overhanging the mouth on the anal side. The retraction of 

 the introvert is effected by a pair of bands of muscular fibres, the 

 parieto-vaginal muscles, passing to it from the body-wall, and 

 by a pair of retractor muscles passing from the latter to the ali- 

 mentary canal. 



