Freshwater Bryozoa, with descriptions of new Species. 183 
described, immediately above them and extending upwards to 
the termination of the cell. These filaments have their outer ex- 
tremities 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 har- 
mony with the contraction of the upper portion of the tentacular 
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 prin- 
cipally 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. II. fig. 2, where the inverted 
lips of the tanic join to the tentacular sheath, it is suddenly con- 
stricted as ii by a powerful sphincter muscle. In fact the whole 
of the tunic is undoubtedly contractile, yet in no part of it have 
1 detected muscular fibres. By the contraction of this lining 
membrane the capacity of the visceral cavity is diminished ; and 
thus by the pressure of the contained fluid the protrusion of the 
polype is effected. This matter however will be discussed more 
fully when we come to speak of this portion of the anatomy of 
Paludicella. 
To understand the combined action of the various sets of 
muscles in Plumatella and Fredericella, we have only to watch 
the animal when about to issue from the cell. The first change 
observed is the contraction of the tunic, Pl. II. fig. 47,7, and 
Pl. III. fig. 4.5, the walls of which are brought nearer together 
towards the lower portion of the cell. The pressure thus occa- 
sioned on the contained fluid compels the polype to begin its 
ascent ; at the same time the sphincter contraction of the upper 
portion of the tentacular sheath relaxes, so that the bundle of 
tentacles can force their way without difficulty. As the polype 
gradually advances upwards the circle of strong radiating muscles 
comes into play, and it is a sight of no little interest to watch 
them drag upon the tentacular sheath, allowing the inferior por- 
tion of it to roll upwards attached to the tentacular disc. As soon 
as the ascent is arrested by these muscles, the sheath being in- 
verted as far as they will permit, protrusion is complete, and the 
tentacles at once assume their proper arrangement. 
