HABITS. 73 



the most important difference, consisting in the greater number of tentacula, is founded on a 

 figure of Paludicella JEhrenberffi, given by myself some years before, and which having been 

 incorrectly engraved with too many tentacles, has thus unfortunately become a source of error 

 in Mr. Hancock's determination of the Northumberland species. 



In the same year, I presented to the British Association a report on the state of our 

 knowledge of the fresh-water Polyzoa,* in which it was my object to give a detailed account 

 of the anatomy of these animals, and a synopsis, with diagnoses, of all the known species ; and 

 in the year 1852, I read, before the Royal Irish Academy, a memoir on the homologies of the 

 organs in the Tunicata and Polyzoa,! in which the fresh-water hippocrepian forms were 

 adduced as affording a means of clearing up some difficult points in the homological relations 

 of the two groups. 



In 1851, we find several communications on the subject of the fresh- water Polyzoa of 

 Pennsylvania, presented by Dr. Joseph Leidy to the Academy of Natural Sciences of Phila- 

 delphia. J In these the author describes some new species of Plumatella, and two new 

 genera {Pectinatella, Leidy, and Uniatella, Leidy) of fresh-water Polyzoa. 



In 1854, Leidy presented to the Academy an additional notice,§ in which, with an 

 amended diagnosis of Urnatella, he confirms the claim of this animal to rank as a distinct 

 genus of fresh-water Polyzoa ; while, at the same time, he describes another new species of 

 Plumatella. Leidy 's account of Urnatella is confined to a simple diagnosis, but the author 

 proposes to give, hereafter, a full description of the genus. Leidy 's communications on the 

 subject of the fresh-water Polyzoa must be regarded as among the most important contribu- 

 tions in a zoographical point of view which have of late years been made to this department 

 of natural history. 



For some years past, I have continued to make the fresh-water Polyzoa the subject of 

 careful study, and the result has been the acquisition of many new facts, and the correction of 

 some errors into which I had previously fallen. The later additions which I have thus suc- 

 ceeded in making to our knowledge of these animals have been hitherto unpublished, and are 

 now, in the present monograph, for the first time made known. 



2. 



Habits of the Fresh-water Polyzoa. 



Besides presenting well-marked differences in form, the fresh-water Polyzoa differ also 

 considerably from one another in their habits. Some delight in the pure clear water of 



* Allman, Report on the present state of our kuowledge of the fresh-water Polyzoa. ' Report of 

 Britisli Association/ 1850. 



■j- Allman, On the Homology of the Organs of the Tunicata and the Polyzoa. ' Transactions of 

 the Royal Irish Academy,' vol. xxii, 1852. 



+ Leidy, in 'Proceedings of the Academy of Natural Sciences of Philadelphia,' vol. v, pp. 261, 



265, 321. 



§ Id., vol. vii, p. 191. 



10 



