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 invagination 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 invaginated to a great extent when it was retracted.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 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 



