BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 387 
difficult however to understand how the act of retraction is 
accomplished ; 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 retractors 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 connection 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invaginated to a great extent when it was retracted.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 being 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 
