PROTRUSION OF THE POLYPIDE 



499 



muscles or its sphincter in a 

 state of contraction in order to 

 remain in that position. And 

 as a matter of fact, CristatcHa, 

 and Lopliopus differ from most 

 other Polyzoa in the readiness 

 with which they expand their 

 tentacles, after they have been 

 induced to retract themselves by 

 mechanical irritation. 



Plumatella, and other forms 

 have a chitinous ectocyst, which, 

 however, is sticky when it is 

 first formed. By virtue of this 

 property, the branches become 

 attached to the leaf on which 

 the colony is growing, and may 

 have their natural transparency 

 obscured by taking up foreign 

 bodies. The stiffness of the 

 ectocyst naturally involves some 

 modification of the process by 

 which the polypides are pro- 

 truded. In some cases, this is 

 effected by the separation of the 

 endocyst from the ectocyst in 

 the lower parts of the tvibe. 

 The muscles of the body-wall 

 can thus press on the fluid of 

 the body -cavity without being 

 restrained by the inflexible ecto- 

 cyst. In other cases, the tube 

 (jf ectocyst is rendered flexible 



by the presence of a thin line Flo 2W.I>l 



along one side where the chitin 

 is deficient. 



The upper end of the re- 

 tracted tentacle -sheath is con- 

 nected with the body -wall by 

 bands known as the parieto-vaginal muscles (Fig. 249, ?>). These 



mnatella repens L., R. Tare, 

 X 30. a, Anus ; b, polypide-bud ; c, 

 caecum of stomach ; d, duplicature ; e, 

 epistome (see p. 476) ;/, funiculus ; 

 <j, ganglion ; , retractor muscle ; p, 

 parieto-vaginal muscles ; ph, pharynx ; 

 y, statoblasts attached to/. 



