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, 

 PI. II. fig. 1 k, in Plumatella and Fredericella. It is situated just 

 below the entrance to the oesophagus 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 oesophagus ; another that comes from the lower ex- 

 tremity of the ganglion may also be seen passing downwards 

 close to the oesophagus. 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, PI. II. figs. 2 g, g & 4/, 

 and PI. III. figs. 4 g & 5 i, 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 oesophagus. 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, 

 PI. II. fig. 4#, 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 the polype back into the cell ; and when it is so withdrawn, the 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, is composed 

 of a circle of stout, isolated, radiating fibres, all placed in the 



