FLUVIATILE POLYZOA. 475 



simply rotate upon their axis, or they tumhle over and over ; or, 

 selecting a fixed point, they whirl round it in rapid circles, carrying 

 every loose particle with them ; others creep along the bottom of the 

 watch-glass upon one end, with a waddling gait ; but generally, after a 

 few hours, all motion ceases, and they are found to have attached them- 

 selves to the bottom of the glass. At the expiration of forty-eight 

 hours, the rudiments of a cell are observable, extending beyond the 

 margin of the body ; but any account of their further development is 

 still a desideratum. 



(1243.) We have hitherto only spoken of those Polyzoa whose 

 habitat is the sea ; besides the marine genera, there are, however, many 

 individuals belonging to this class that abound in fresh water. The 

 polyparies of the FLTJVIATILE POLYZOA are met with in ponds and 

 streams, adherent to any foreign bodies which may be casually sub- 

 merged*. Thus, they are found attached to stones at the bottom of the 

 water ; upon the shells of Anodon, Unio, and other freshwater mollusca; 

 upon leaves, more especially those of the Water-lily (Nymphceo) and 

 of the Bistort (Polygonum amphibium) ; upon floating wood ; upon the 

 stems of Arundo phragmites and of various other plants. Some genera 

 (Alcyonella and Fredericella) frequently agglomerate into masses of con- 

 siderable size, such as might be mistaken for spongillge. The Paludi- 

 cellce often form an inextricable interlacement of filaments, spread out 

 over shells and stones. Cristatella and Lophopus are generally met 

 with upon the stem of some aquatic plant, such as the Brook-lime ( Vero- 

 nica beccabunga), resembling, when examined by the naked eye, a layer 

 of fluid albumen, which might easily be mistaken for the eggs of Lim- 

 nceus stagnates. In order to examine these animals in a living state, it 

 is necessary to leave the leaf to which they are attached for some time 

 undisturbed in a glass of clear water, when they will soon be seen spread- 

 ing forth their beautiful tentacula as they protrude from their delicate 

 cells. By frequently changing the water, more especially if it is rich in 

 Naviculce and Bacillarice, they may be kept alive for months, affording 

 objects of continual interest for the microscope. 



(1244.) In the freshwater Polyzoa the structure of the external enve- 

 lope is similar to that of the marine species, except that in no instance 

 are the fluviatile genera known to possess a calcareous polypary. 



(1245.) In Cristatella mucedo^ (fig. 241, 3) the polypary or external 

 envelope (d) is membranous and slightly cordiform, its surface is 

 tuberculated, and it is incapable of contraction. In this outer covering 

 several individuals are contained; but although produced from one 



* " Recherches stir les Bryozoaires fluviatiles de Belgique," par P. J. Van Beneden 

 (Nouv. Mem. de 1'Acad. de Bruxelles, 1847). 



t M. Turpin, " Etude microscopique de la Cristatella mucedo, espece de polype 

 d'eau douce " (Ann. des Sci. Nat. for 1837). Also, another memoir upon the same 

 subject by M. P. Gervais (ibid.}. 



