No. 7.] OSLER — FRESH- WATER POLYZOA. ol>9 



ON CANADIAN FRESH-WATER POLYZOA. ^^^ 



]]v William O.slkk. M.D. 



The Polyzoa, or Bryozoa as they are sometiiiies called, form 

 an exceedingly interesting group of animals. From their exten- 

 sive distribution in iieoloaieal formations and from the abund- 

 ance and great beauty of the marine species at the present day, 

 they have attracted an unusual share of attention from naturalists, 

 while the elegance and plant-like appearance of many of the 

 forms make them at the sea shore and in the museum favorites 

 with the public. For a long time the Polyzoa were classified 

 with the hydroid polyps among the Radiata, and even by Owen, 

 in 1855, this method was adopted. Dr. J. V. Thompson, in 

 1830, was the first to separate them and apply the name Polyzoa 

 to the whole class. At present they are classified as the lowest 

 division of the Mollusca, forming together with the Tunicates and 

 Brachiopods the class Heterobranchiata in the old system, or 

 the division Molluscoida in the new. The Polyzoa are divided 

 into two orders, 1st, the Phylactolasmata, in which the tentacles 

 are arranged in the form of a horse shoe or crescent, and which 

 are provided with a valve guarding the throat. 2nd, the Gym- 

 noloemata, in which the tentacles are arranged in a circle, and the 

 throat is not provided with a valve. In the Phylactolgemata 

 there are three sub-borders, of which the first, Lophopea, contains 

 almost all the fresh-water species. Prof. Allman divides the 

 Lophopea into two great families, the Cristatellidi^ and the Plu- 

 matellidse — in the former the animal is locomotive, in the latter 

 fixed. The genera in which Canadian species occur, as far as 

 we know at present, are Cristatella, Plumatella and Pectinatella, 

 and the five species which I have identified Cr. opliidioldea ^ PL 

 diffusa, aretlmsu and vitrca, and Feet, magiiljica. 



As I have nothing new upon this subject to bring forward, 

 I shall proceed to make some general remarks upon the structure 

 and life history of these creatures, and demonstrate the specimens 

 on the table before you. I may as well here explain one or 

 two terms which will be frequently used in the descriptions. 

 The term ccenaclani when employed indicates the common 

 system and solid basis of the animal. The external coating is 



* Read before the Xatural Historv Society 



