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MONOGRAPH 



THE FRESH-WATER POLYZOA. 



Among the most beautiful and interesting forms of invertebrate animals are those strange 

 phytoidal productions which, long confounded with the polypes, were at last, by the nearly 

 simultaneous investigations of several naturahsts, separated as a distinct group, and described 

 by Thompson under the name of Polyzoa, and shortly after indicated by Ehrenberg under that 

 of Bryozoa. They are chiefly inhabitants of the sea, where they may be witnessed under 

 numerous plant-like guises ; now spreading like a lichen over submerged stones, or old shells, or 

 the broad fronds of Laminaria and other sea-weeds ; now forming soft, irregular, fungus-like 

 masses, or hard, calcareous, branchy growths, like diminutive trees ; and now again presenting 

 the appearance of the most delicate and exquisitely formed sea-weed or moss, offering, even to 

 the unassisted eye, in the endless repetition of the same element of form, objects of surpassing 

 symmetry and beauty. 



The Polyzoa, however, are not by any means exclusively confined to the ocean ; and though 

 by far the greater number are marine, yet in the still and running waters of the land — in the 

 broad river and the rushing stream — in the pure, cold mountain lake and the stagnant waters of 

 the moory fen, species are to be found, which in interest yield not one jot to their brethren of 

 the sea, and offer to the naturalist an inexhaustible source of gratification, in the beauty of 

 their forms and the wonders of their organization. 



It is to these fresh-water species, which, independently of their peculiar habitat, possess 

 certain characters entithng them to be viewed as a group apart from the marine representatives 

 of the class, that the present work is to be devoted. It will be well, however, before 

 entering into the detailed treatment of our subject, to take a general historical view of the 

 facts which led to the establishment of the Polyzoa as a distinct class of the animal kingdom. 

 These facts are so intimately mixed up with the gradual development of correct views as to 

 the nature of the true Polypes, with which the Polyzoa had, until recent times, been con- 

 founded, without any suspicion of the wide interval by which they were really separated, that 

 their historical statement will necessarily involve a rapid glance at the progressive steps made 

 by the earlier naturalists in the determination of the animality of corals. 



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