DR. A. FARRE ON THE STRUCTURE OF THE C I LI OB RAC H I ATE POLYPI. 397 
rietes upon which they were arranged was seen to recede from the inner surface of the 
cell, and to be drawn into longitudinal lines, especially at their points of origin and 
insertion. When the animal retired they returned to their former dimensions. These 
parietal muscles, which in structure exactly resemble the retractors, I have observed 
in every species of ciliated polype that has since come under my notice, and have 
ascertained its existence in Bowerbankia. (Plate XXI. fig. 13.) 
These transverse filaments then, acting together from top to bottom of the space 
upon which they are arranged, must necessarily tend by their contraction to diminish 
considerably the diameter of the visceral cavity, and will therefore 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, and it might be supposed that this 
would satisfactorily account for the act of protrusion ; but it must be remembered 
that the contents of the cavity are folded up in a complicated manner, and the cell 
closed by its operculum, the whole of which parts have to be unfolded in regular 
order before the act of expansion is completed. I doubt, therefore, whether this 
simple apparatus could accomplish this act unassisted; but I believe it to be mate- 
rially aided by the cooperation of the alimentary canal, which undoubtedly has the 
power of straightening itself from the sigmoid flexure into which it is thrown when 
the animal is retracted. I am led to think this from having frequently observed the 
great extent of motion which the upper part, especially of the alimentary canal, is 
capable of exercising, independently of any action of the muscles attached to it; and 
from having also noticed occasionally that, during the rising of the tentacula, the 
unfolding of the integument from around them seemed rather to follow as the con- 
sequence of their advance, than as being the means of effecting it, which cannot well 
be explained, if we suppose the fluid of the body to be driven upwards by the contrac- 
tion of the parietal muscles with sufficient force of itself to expand the upper part of 
the body, and so to carry up the alimentary canal and to thrust out the arms. And 
this appears the more probable when we observe that even in 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 polypes possessing no distinct muscles to assist 
in these operations. 
To return then to Bowerbankia. Let us see how far these considerations will ap- 
ply to the explanation of the phenomenon in question. The animal being retracted, 
with the stomach resting upon the bottom of the cell, begins to erect itself by straight- 
ening the alimentary canal ; and the tentacula must be the parts first to rise. Before, 
however, these can protrude from the cell it is necessary that the flexible operculum 
which closes the mouth of it should be unfolded. As there does not appear to be 
any separate apparatus for this purpose, and as I have never observed it to occur in- 
dependently of the motions of the animal, it may be presumed that this is effected by 
the pressure from below when the animal endeavours to rise. 
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