POLYP1FERA. 



The filaments have a watery transparency and 

 smooth surface, and under the highest powers 

 of the microscope present neither an appear- 

 ance of cross-markings, nor of a linear arrange- 

 ment of globules. These muscles, though 

 apparently attached to the inner walls of the 

 cell, must yet have the membranous parietes 

 of the body interposed between their inser- 

 tions and these walls. In the lower part the 

 integument is only occasionally seen separate 

 from the walls of the cell, but above it may 

 be easily discerned in the expanded animal, 

 passing up to be inserted around the tenta- 

 cular ring, and thus distinctly bounding this 

 part of the body, which is always free within 

 the expanded operculum. 



The operation of this mechanism in retract- 

 ing the animal within its cell is as follows. 

 The tentacula, from being expanded in the 

 form of an inverted cone, are brought together 

 into a straight line, and immediately begin 

 to descend (fig. 60, d). Their descent is 



Fig. 60. 



Bowerbankia densa, magnified 80 diameters. A series 

 to show the mode in which the operculum and upper 

 part of the body is unfolded. The same animal is 

 represented in four different stages. 



a. First stage: the top of the cell completely 

 closed ; the seta folded up in the centre (1), with 

 the flexible portion of the cell (2) inverted and closely 

 surrounding them ; the muscles contracted (3, 4.) 



b. Second stage : the bundle of the setae (1) 

 rising from the centre of the cell being forced up- 

 ward by the pressure of the tentacula ; the flexible 

 portion (2) rolling from around the setae, and the 

 muscles (3) put upon the stretch. 



c. Third stage : the flexible portion (2) completely 

 everted; the setas (1) still lying together; the 

 tentacles just appearing between them. 



d. Fourth stage : the tentacula appearing above 

 the margin of the operculum ; the integument of 

 the body, which fonns the tentacular sheath, half 

 everted (3); the operculum completely expanded. 

 (These stages are taken arbitrarily, the process 

 being continuous.) The animal is shown completely 

 extended at fig. 56, a. (After Farre.') 



effected by the contraction of the muscle 

 (fig. 56. 9) which passes from the base of 

 the cell to the tentacular ring, whilst at the 

 same time the stomach is drawn down by its 

 retractor (fig. 56. 8). The whole body, how- 

 ever, does not descend in a mass, but must be 

 folded up in a somewhat complicated manner, 

 in order that the cell may completely enclose 



it. For this purpose the oesophagus sur- 

 mounted by the tentacula descends first, whilst 

 the integument of the upper part of the body 

 begins to be inverted at the point where it has 

 its insertion around the tentacular ring. As 

 the descent of the tentacula proceeds, the 

 inversion of the integument continues forming 

 a sheath around them (fig. 60, c), until the 

 extremities of the arms have descended to a 

 level with the top of the unyielding portion 

 of the cell. The animal is now drawn com- 

 pletely in, the stomach brought close to the 

 bottom of the cell, and the oesophagus bent 

 in the form of the letter S ; the tentacula 

 lying straight in the axis of the cell, enclosed 

 in their tegumentary sheath, and so sepa- 

 rated from the fluid in the general visceral 

 cavity, the centre of which they have the 

 appearance of occupying, while in fact they 

 are external to it. The animal being thus re- 

 tracted, the next step of the process is to 

 draw in the upper part of the cell after it. 

 This process, however, always commences 

 before the retraction of the body is com- 

 pleted, and by the time that the ends of the 

 arms are on a level with the base of the 

 setae, the latter are brought together in 

 a bundle, and begin to descend apparently 

 by the action of the lower of the two sets of 

 opercular retractors above described. Their 

 descent, like that of the tentacles, takes place 

 exactly in the axis of the upper part of the 

 cell, and is accompanied by an inversion around 

 them of its flexible portion, similar to that of 

 the integument of the body around the tenta- 

 cula during their descent (fig. 60, b). Whilst 

 the lower set of muscles are drawing down 

 the set(E y the upper set complete the retraction 

 of the flexible part, and the whole operculum 

 is thus packed closely in the upper part of 

 the cell, the end of which now presents a 

 triangular indentation, corresponding with the 

 triangular arrangement of the opercular re- 

 tractors (fig. 60, ). Thus the whole pro- 

 cess of retraction may be easily accounted 

 for, and the office of each set of muscles 

 satisfactorily explained ; but the protrusion of 

 the animal is effected by a totally different 

 mechanism, viz., by the action of a set of 

 transverse muscles acting upon the lining 

 membrane of the cell, so as by their contrac- 

 tion to diminish considerably the diameter of 

 the visceral cavity, and consequently exercise 

 a pressure upon the fluid which it contains. 

 The effect of this will be to elongate the body 

 in the direction in which it is most free to 

 move ; but Dr. Farre supposes that the act 

 of protrusion is materially assisted by the co- 

 operation of the alimentary canal, which un- 

 doubtedly has the power of straightening 

 itself from the sigmoid flexure into which it 

 is thrown when the animal is retracted ; and 

 that this is the case appears the more pro- 

 bable, when we reflect that in the case of the 

 simple hydriform polypes the advance and 

 receding of the animal in its cell is entirely 

 effected by the action of the parietes of the 

 body, which are analogous to the alimentary 

 canal in the present case, the hydriform po- 



E 3 



