XXXll INTRODUCTION. 



dwelling, the parietal muscles (see page xiii) are at once 

 brought into play, and contract the endocyst or lining 

 membrane, thus reducing the perigastric space, and com- 

 pressing the fluid, which immediately begins to act on the 

 body of the polypide, and to force it upward in the direc- 

 tion of the orifice*. As it rises, under the influence of 

 this pressure, the sphincter which guards the entrance 

 gives way, and the tentacular pencil passes outwards. 

 Then the parieto-vaginal muscles relax their hold on 

 the inverted sheath, and permit it to follow the course of 

 the advancing corona. The wall-muscles maintain their 

 tension, and keep up a constant pressure from below; 

 steadily and rapidly the ascent continues, the alimen- 

 tary canal unfolding the while, until, the sheath being 

 fully evaginated and the tentacles freed from restraint, 

 the exquisite bell suddenly expands and the cilia start into 

 play. 



Although there can be no doubt that the parietal 

 muscles are thus the great agents in effecting the pro- 

 trusion of the polypide from its cell, I am quite inclined 

 so far to agree with Dr. Farre as to hold that a certain 

 subsidiary part is played in the process by the muscular 

 walls of the alimentary canal itself; that it cooperates to 

 some extent with the mechanical action of the compressed 

 fluids. 



* " These fibres [the parietal] were distinctly seen to contract whenever 

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

 its retiring into ita cell, the walls of the latter being so pellucid that the 

 minutest alteration in the form of these muscles was readily seen." FAKIU.. 



The contraction of the cavity of the cell, which takes place durinp the 

 expansion of the polypido, is very apparent in such Ctenostomatous forms 

 as have a yielding membranous area on one side of the zooecium. In Mi- 

 mosella, for instance, when the polypide issues, this portion of the cell-wall 

 is drawn inwards, and a distinct cleft is formed ; on its- return it fills out 

 again, and its presence is with difficulty detecled. 



