Freshwater Bryozoa, with descriptions of new Species. 181 
a ganglion in these animals, and has more recently ascertained 
the existence of nerves. I have also detected a large ganglion, 
Pl. II. fig. 1 &, in Plumatella and Fredericella. It is situated just 
below the entrance to the cesophagus on the external surface, close 
to the base of the tentacles and just above the anal orifice. It is 
therefore placed between the oral arms in Plumatella, and in 
Fredericella at the corresponding point. In the latter I have 
observed two or three nerves passing from the ganglion upwards 
in the direction of the tentacles, and one apparently going to 
embrace the cesophagus ; another that comes from the lower ex- 
tremity of the ganglion may also be seen passing downwards 
close to the esophagus. This is all I have been able to make 
out respecting the nervous system, though undoubtedly more is 
to be learnt. 
Muscular System.—There are three distinct sets of muscles in 
Plumatella and Fredericella ; one for the retraction of the polype, 
another to assist in the act of protrusion, and the third probably 
accessory in closing the orifice. The first and most conspicuous 
set of muscles, the polype retractors, Pl. II. figs. 29,9 & 4f, 
and Pl. III. figs. 4 g & 5 3, i, is divided into two equal bundles, 
one passing on each side of the polype. These bundles are com- 
posed of numerous, stout, isolated fibres, having their origin in 
the walls of the cell a considerable way below the retracted po- 
lype; and passing upwards have their superior extremities in- 
serted at the tentacular disc or base of the oral arms and at the 
upper portion of the cesophagus. There are also two similar 
bundles of muscular fibres in Alcyonella, but in this genus they 
have their origin at or close to the bottom of the cell. When the 
animal of Plumatella is exserted, two or three of the stoutest, 
Pl. II. fig. 4.9, of these fibres are seen to be attached on each 
side further forward than the rest at the base of the oral arms. 
Also in Fredericella similar fibres are inserted at the correspond- 
ing parts of the tentacular disc. These stout fibres have their 
origin a little lower down the cell than the rest. 
The function of these two bundles of muscular fibres cannot 
for a moment be mistaken: they are for the purpose of drawing 
the polype back into the cell; and when it is so withdrawn, the 
fibres of this, the most powerful muscle of the animal, may be 
seen in a relaxed state and bent upon themselves in a loose un- 
dulating manner about the tentacular disc and downwards to 
their origin. The few strong fibres alluded to undoubtedly as- 
sist in the retraction of the polype; but are also apparently the 
principal agents in rotating the head, so to speak, of the polype 
when exserted. 
The second set of muscles, PI. II. figs. 1 & 2 m, h, 1s composed 
of a circle of stout, isolated, radiating fibres, all placed in the 
