BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 379 
ments.* We thus clearly see that this set of radiating muscles 
is a compensation for the deficiency of the circle of sets 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 
described, immediately above them and extending upwards to the 
termination of the cell. These filaments have their outer 
extremities attached to the inner surface of the tunic; and con- 
verging towards the axis of the cell, their inner extremities are 
attached to the upper portion of the tentacular sheath and the 
inverted margin of the tunic. These fibres are equally numerous 
and fine in both Plumatella and Fredericella, and appear to be 
for the purpose of assisting in closing the orifice, acting in 
harmony with the contraction of the upper portion of the tenta- 
cular sheath and the inverted lips of the orifice. They may, acting 
in the opposite direction, also assist in opening the channel, but 
the tentacles themselves would appear quite adequate to force a 
passage on the relaxation of the contractions about the orifice. 
The function of these fibres is in fact to keep in unison the tunic 
near the opening, and the upper portion of the tentacular sheath. 
The upper portion of the tentacular sheath and inverted lips 
of the tunic are highly contractile, and it is by their agency 
principally that the orifice is closed when the animal is retracted. 
I have not, however, been able to detect any muscular fibres for 
the purpose, though at the point, Pl. III. fig. 2 7., where the 
inverted lips of the tunic join to the tentacular sheath, it is 
* Dr. Farre has described this apparatus in his paper so frequently referred 
to, but seems scarcely to have arrived at a full knowledge of its function. He 
considers that it is ‘‘ 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 pointed out. The circle of sete is then seen to 
compress the lower portion of the extended polype; and when the tentacular 
disc 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 sete, is strengthened with numerous, 
minute, transverse fibres, forming the whole into a powerful sphincter, thus 
giving great firmness to the part. By this arrangement, Bowerbankia is 
enabled to raise the tentacular disc far above the polype-cell, and yet to remain 
as perfectly under the control of the rotatory and retractor muscles, as is the 
tentacular disc of Fredericella and Plumatella, in both of which it is confined 
close to the orifice of the cell by the action of the radiating muscular fibres. 
