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 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 

 flexible cuticle, and forms an introvert capable of being retracted 

 into the interior of the zocecium. 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 ccelome. They are beset with vibratile 



ref" 



state 



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

 mouth ; it. cesophagus ; repr. reproductive gland ; retr. retractor muscle ; *(. 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 Phylactolsemata (Fig. 258) the 

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

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

 lobe the epistome (Fig. 259 epY 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-i'aginal muscles, passing to it from the body-wall, and 

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

 mentary canal. 



