380 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
suddenly constricted as if by a powerful sphincter muscle. In 
fact the whole of the tunic is undoubtedly contractile, yet in no 
part of it have I 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. III. fig. 4 7, 7, and Pl. IV., fig. 4 
b, the walls of which are brought nearer together towards the 
lower portion of the cell. The pressure thus occasioned, 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 portion of it to roll up- 
wards attached to the tentacular disc. As soon as the ascent is 
arrested by these muscles, the sheath being inverted as far as they 
will permit, protrusion is complete, and the tentacles at once 
assume their proper arrangement. 
The muscular apparatus of Paludicella differs in some respects 
from that of Plwmatella and /redericella. In the former there are 
six sets of muscles—three in connexion with retraction, two with 
protrusion, and one for closing the orifice on the retreat of the 
polype. Of the retractors, one set acts directly upon the animal, 
the other upon the tubular orifice of the cell. The former set, 
Pl. V. fig. 1 0, the most powerful in the animal, is similar to the 
tentacular retractors of Dr. Farre : it differs. only from the polype- 
retractors in Plumatella, and the other genera already spoken of, 
in not being divided into two bundles. It is composed of 
numerous, stout, long, linear fibres, originating from the inner 
surface of the anterior wall of the cell, more than half-way down ; 
