500 POLYZOA 



serve not only to dilate the orifice when protrusion is commenc- 

 ing, but also to prevent the polypide from being forced out too 

 far. They are arranged in such a way that a circular fold, the 

 duplicature (d), is never turned inside out, even in the state of 

 complete protrusion of the polypide. 



The mechanism of the protrusion of the polypide in the 

 Gymnolaemata is in many cases obscure. The body-wall is not 

 muscular in this group, in some forms of which, however, short 

 strands known as the parietal muscles (Fig. 234,^?) pass across 

 the body -cavity from one point to another of the zooecium. 

 As doubts have been thrown on the function of these muscles in 

 causing protrusion, it will be worth while to refer to the detailed 

 and convincing statements of Farre, 1 relating to this point. 



Farre's observations were made on certain transparent Cteno- 

 stomes (Bowerbankia and Farrella). He states that the parietal 

 muscles * were distinctly seen to contract whenever the protrusion 

 of the animal took place, and to become relaxed again upon its 

 retiring into its cell." Their contraction may indent the outline 

 of the ectocyst, or may cause the separation of the endocyst from 

 the ectocyst. The endocyst is then drawn into longitudinal lines 

 at the origin and insertion of these fibres. It is further suggested 

 that some part is played in the process by the muscular walls of 

 the alimentary canal, which is a good deal bent in the retracted 

 condition. The effort to straighten itself is believed to have 

 some share in forcing out the polypide. The flexible, membran- 

 ous character of the "aperture" (see p. 524) in Membranipora 

 (Fig. 256, A) is said by Mtsche 2 to be an arrangement for the 

 protrusion of the polypides ; the parietal muscles passing from 

 the lateral walls of the zooecium to the upper membranous wall, 

 which is accordingly depressed by their contraction. 



Although it is hardly possible to doubt the accuracy of Farre's 

 observations, which have, moreover, been confirmed by Hincks, it 

 is by no means certain that this is the whole explanation in all 

 cases. Oka, 3 for instance, states that protrusion of the polypide 

 in Phylactolaemata can be effected in a branch whose body- wall 

 has been cut open. Pergens 4 believes that the diaphragm (Fig. 



1 Phil. Trans. 1837, p. 396. 



2 Zcitschr. wiss. Zool. xxi. 1871, p. 426. 3 J. Coll. Japan, iv. 1891, p. 113. 

 4 Zool. Anz. xii. 1889, p. 508. This paper contains references to M. Jullien's 



writings on the mechanism of protrusion. 



