550 



POLYZOA. 



the alimentary canal and tentacles are called the polypide (for 

 origin of this absurd nomenclature see below, p. 555). The cell 

 at, or near, its anterior end has an opening called the orifice, 

 through which the anterior part of the zooid can be protruded and 

 retracted. This anterior invaginable part of the body, which carries 

 a crown of tentacles, is also covered by cuticle, which is continuous 

 with the stiff er ectocyst at the orifice, but distinguished from the 

 latter by its softness and flexibility. At the orifice the body-wall 

 (with its soft cuticle) is invaginated inwards, and passes thence on 

 to the anterior and extensible part of the body. In many Polyzoa 



this reduplication of the body- 

 wall is present even when the 

 zooid is protruded (Fig. 440). 

 The greater part of the anterior 

 region of the body, with its 

 crown of tentacles, can however 

 be protruded from the cell and 

 retracted into it again by special 

 muscles the parieto-vaginal and 

 the retractor muscles which tra- 

 verse the body-cavity (Fig. 440). 

 That part of the body-wall which 

 encloses the space in which the 

 tentacles lie when completely 

 retracted is called the tentade- 

 slieath. 



In the Cheilostomata there is 

 a movable chitinous lid, the 

 operculum, \vhich can shut down 

 over the orifice. Sometimes the 

 ectocyst is raised into a ridge 

 the peristome round the orifice : 



the tube thus formed is the secondary orifice. Zoarium is the term 

 applied -to the whole colony. The term ooecium or ovicell is 

 applied to the receptacles in which the ova undergo their develop- 

 ment. The ooecia are of very different kinds : in the Cheilo- 

 stomata they are merely pouches of the zooecia into which the eggs 

 pass. In some if not all Cyclostornata (Crisia) they are special 

 zooecia in which the polypide is rudimentary or degenerates. This 

 is probably the case with all ovicells in the Cydostomata, but it has 

 not been proved in all cases. In Pliylactolaemata, in which the 



FIG. 440. Plnmatella repens (after Allman). 

 T tentacles ; L lophophore ; Oe oeso- 

 phagus ; A anus ; Mg stomach ; F funi- 

 culus ; St statoblasts ; Ts tentacle 

 sheath ; Ek ectocyst ; En endocyst ; 

 Gg ganglion ; Pvm parieto-vaginal mus- 

 cles ; Rm retractor muscle. 



