188 Mr. A. Hancock on the Anatomy of the 



cell, two in front immediately below where the tube joins the cell, 

 and two behind in a line with the upper wall of the tube ; hence 

 the fibres are placed in front of and behind the polype, and are 

 inserted into opposite points of the tentacular sheath a little way 

 below its summit, having on each side of them the two bundles 

 of the tube-retractors. In the retracted state of the polype these 

 fibres are seen passing downwards towards their insertion. When 

 the polype is protruded these muscles cause the sheath to double 

 upon itself, and thus retain a portion of it within the tube ; but 

 not to the same extent as in Plurnatella and Fredericella. It has 

 already been pointed out that in these genera this set of muscles 

 compensates for the want of the circle of setae which surmounts 

 the orifice in the marine species. In Paludicella, however, we 

 have already seen that there is a wide, delicate, membranous cup 

 which rises from the inner surface of the tube a little within the 

 orifice. This cup is undoubtedly the homologue of the circle of 

 setae alluded to, but in a very rudimentary state, and probably of 

 little or no functional utility : consequently these muscles are 

 still present, though, as might be expected, not so fully developed 

 as in those genera entirely deprived of this appendage. 



We have now gone through the whole of the muscular appa- 

 ratus for retraction and propulsion, -and to verify the use of the 

 various sets of muscles, we must once more observe the animal 

 while issuing from the cell. The first symptom indicative of the 

 polype's inclination to come forth is the contraction of the parietal 

 muscles, causing the tunic in certain places to leave the walls of 

 the cell, particularly towards the lower portion ; on this the polype 

 commences to move up the cell, and at the same instant the tube- 

 retractors relaxing the inverted lips of the orifice begin to be 

 evolved, and as the contraction of the parietal muscles goes on 

 the polype advances upwards, and more and more of the tube is 

 turned out, in the manner of the eversion of the horn of the com- 

 mon snail ; at length the membranous cup makes its appearance, 

 not doubled upon itself, but in an erect position — the margin first, 

 just as the circle of setee is exserted in Bowerbankia. The cup at 

 first is laterally compressed, having been packed longitudinally 

 in the axis of the tube : the tips of the tentacles now emerge 

 through the centre of this cup, and as they pass upwards pressed 

 together in a line side by side, its lateral folds give way, and by 

 the time that the tentacular disc has reached the mouth of the 

 cell, the cup is perfectly expanded. The muscles preventing the 

 entire eversion of the tentacular sheath may now be seen in ac- 

 tion near the upper extremity of the tube, holding back the mem- 

 branous sheath and causing it to roll upon itself. The polype is 

 now fairly above the mouth of the cell, and as the tentacles ex~ 

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