Freshwater Bryozoa, with descriptions of new Species. 189 
pand it has attained its greatest elevation; the cilia then com- 
mence to play, and all kinds of particles are hurried towards the 
mouth. 
The retraction of the polype is instantaneous, so rapid indeed 
that it is quite impossible to follow with the eye the actions of 
the muscles ;—such is the velocity with which this feat is per- 
formed, that from complete protrusion to mvagination nothing 
can be perceived but the settling of the polype upwards, after 
having apparently been dragged too far down the cell. It is not 
difficult however to understand how the act of retraction is ac- 
complished ; the operation of the muscles will be reversed. First 
the parietal muscles must relax, allowing the tunic to assume its 
place close to the cell-walls ; at the same instant the polype-re- 
tractors will contract, and as the animal sinks into the cell the 
superior tube-retractors will also contract ; next the inferior tube- 
retractors will come into play; and finally, after retraction is 
complete, the sphincters will close the orifice. 
On comparing the muscular system of the freshwater Bryozoa 
with that of the marine forms, a great similarity is observed ; 
some interesting modifications however are deserving of notice. 
The most remarkable of these are found in connexion with the 
orifice. In Plumatella and Fredericella there is no tubular in- 
version on the retreat of the animal ; the tunic is certainly doubled 
upon itself for a short distance within the orifice, but it remains 
permanently so. Paludicella on the contrary has the walls of the 
tubular orifice invaginated to a considerable extent when the 
polype is retracted, and when protruded nearly the whole is 
evolved. But Bowerbankia and other marine forms differ from 
the freshwater species in having the mouth of the cell completely 
unrolled when the polype is protruded, the same having been 
invaginated to a great extent when it was retracted. Thus in 
the first and last modifications we see the extremes of variation, 
and consequently the most extensive alterations in the muscular 
arrangements of these parts. Paludicella bemg in a middle state 
has the muscular apparatus to some extent of both ; and in this 
respect connects the freshwater with the marine forms. 
The tube-retractors are wanting in Plumatella and Fredericella, 
and are present in Paludicella and in all the marine species, 
being most developed in the latter. Neither in these nor in 
Paludicella, however, is there anything like the small radiating 
muscles near the orifice in Plumatella and Fredericella ; and the 
marine species, too, are destitute of the large radiating muscles 
in connexion with the tentacular sheath. These, though present, 
we have seen are less developed in Paludicella than in Plumatella 
and Fredericella, the former resembling Bowerbankia in having 
a cup at the mouth of the cell. The polype-retractors are very 
