378 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
the polype back into the cell ; and when it is so withdrawn, the 
fibres of this, the most powerful muscle of the animal, may be 
seen in a relaxed state and bent upon themselves in a loose un-. 
dulating manner about the tentacular disc, and downwards to 
their origin. The few strong fibres alluded to, undoubtedly 
assist in the retraction of the polype; but are also apparently 
the principal agents in rotating the head, so to speak, of the 
polype when exserted. 
The second set of muscles, Pl. III. figs. 1 & 2 m, h, is composed 
of a circle of stout, isolated, radiating fibres, all placed in the 
same horizontal plane, considerably apart from each other, and 
attached by their outer extremities to the inner surface of the 
tunic, some way below the opening; their inner extremities 
converging towards the tentacular sheath, are attached to it 
about one-third from its superior termination. Plwmatella has 
fifteen or sixteen of these fibres, /redcricella about fourteen. 
Their arrangement is perfectly symmetrical. They are for the 
purpose of preventing the inversion of the whole of the tentacu- 
lar sheath, on the protrusion of the polype; and thus to confine 
the oral extremity within a convenient distance above the mouth 
of the cell. 
The true value of these muscles will be fully understood if we 
refer to the marine genus Lowerbankia, in which they are 
deficient, and of course the tentacular sheath can be completely 
inverted, and accordingly the animal is enabled to reach toa 
greater distance than it could otherwise have done. But an ap- 
paratus of extraordinary beauty is provided to obviate the incon- 
venience that must have arisen from the great elevation of the 
tentacular dise, above the support of the horny cell. This is 
effected by what may be considered an elongation upwards of the 
cell. Numerous setz bound together by a membrane, are 
attached to the lips of the orifice, so that when the polype is 
exserted, they stand up in a circle surrounding the lower part of 
the exposed portion of the animal, and give support to it. By 
this means, the far-outstretched tentacular disc is brought com- 
pletely under the control of the muscles for directing its move- 
