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PHYLUM MOLLUSCOIDA 



343 



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 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 



FIG. 286. Cristatella mucedo. Entire colony. (After Allman.) 



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

 for prehension. In the Phylactolsemata (Fig. 285) the lopho- 

 phore is horse-shoe-shaped, in the Gymnolaemata (Fig. 281) 

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

 lobe, the epistome (Fig. 287, ep.) 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. 



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