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. 2n, where the inverted 

 lips of the tunic join to the tentacular sheath, it is suddenly con- 

 stricted as if 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, PI. II. fig. 4 j,j, and 

 PL III. fig. 4 b, 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. 



