182 Mr. A. Hancock on the Anatomy of 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 mner extremities con- 
verging towards the tentacular sheath are attached to it about 
one-third from its superior termination. Plumatella has fifteen 
or sixteen of these fibres, Fredericella about fourteen. Their ar- 
rangement is perfectly symmetrical. They are for the purpose 
of preventing the inversion of the whole of the tentacular 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 Bowerbankia, in which they are defi- 
cient, and of course the tentacular sheath can be completely in- 
verted, and accordingly the animal is enabled to reach toa greater 
distance than it could otherwise have done. But an apparatus 
of extraordinary beauty 1s provided to obviate the inconvenience 
that must have arisen from the great elevation of the tentacular 
disc above the support of the horny cell. This is effected by 
what may be considered an elongation upwards of the cell. 
Numerous setee 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 completely under the 
control of the muscles for directing its movements*. We thus 
clearly see that this set of radiatimg muscles is a compensation 
for the deficiency of the circle of setz in the freshwater polypes. 
The third set of muscles, figs. 1 & 2 n, 2, consists of numerous, 
separate, fine thread-like filaments placed considerably apart, 
without order, but in the same radiating manner as those last 
* Dr. Farre has described this apparatus in his paper so frequently re- 
ferred to, but seems scarcely to have arrived at a full knowledge of its fune- 
tion. He considers that it 1s “‘ for allowing of the freest possible motion to 
the upper part of the body in its expanded state, to which it affords at the 
same time support and protection.” On examining the animal in action it 
is evident that the use of the apparatus is as I have pomted out. The circle 
of setze is then seen to compress the lower portion of the extended polype ; 
and when the tentacular dise moves from side to side the neck always bends 
from the top of the sete at a decided angle, and does not gradually arch 
away from the lips of the cell as might be expected were this contrivance 
+ for the purpose of giving flexibility. The delicate membrane uniting the setze 
is strengthened with numerous, minute transverse fibres, forming the whole 
into a powerful sphincter, thus giving great firmness to the part. By this ar- 
rangement Bowerbankia is enabled to raise the tentacular dise far above the 
polype-cell, and yet to remain as perfectly under the control of the rota- 
tory aud retractor muscles as is the tentacular dise of Fredericella and Plu- 
matelia, in both of which it is confined close to the orifice of the cell by the 
action of the radiating muscular fibres, 
