GILLS. 125 



and other matters forming their food, from the sur- 

 faces upon which they grow. And if some water- 

 snails are placed in a glass jar the inside of which is 

 covered with confervoid growths, the curious patterns 

 left after the action of the snails' tongues will be 

 found to present a very curious appearance. 



Gills. The gills or "beards" of the oyster or mussel 

 exhibit very strikingly the phenomenon of ciliary mo- 

 tion. The gills (branchiae) are respiratory organs, 

 consisting of folds of the skin, covered with cilia, by 

 means of which the water in which the animal lives 

 is set in motion, and constantly changed to aerate 

 the blood within them. The currents thus induced 

 serve also to bring the food which floats in the water 

 towards the mouth of the animal. By snipping off 

 a thin portion of one of the brown beards of a fresh 

 oyster, laying it upon a slide, adding a drop of the 

 ( ' liquor " contained within the shell, and lightly pressing 

 a cover upon the whole, the remarkable phenomenon 

 to one who has not before viewed it will be seen 

 under a somewhat high power about J-inch. The 

 whole field will appear in motion, and the lashing or 

 whip-like action of the cilia will be seen, especially 

 towards the edges of the bars (PI. IX. fig. 36) of the 

 gills. The rapid motion of any floating particles 

 present will also be noticed, showing the direction of 

 the currents of liquid, which, as the liquid is trans- 

 parent, would not otherwise be recognizable. 



BRYOZOA (ffpvov, moss, pwov, animal). The animals 

 included in this Class, which belongs to the Mollusca, 

 are mostly marine. They are microscopic, and con- 

 tained in horny or calcareous sacs or cells, aggregated 

 together to form polyp'idoms (polype, and Sw/,ta, a 

 house) . They are sometimes plant-like or leafy (PL IX. 

 fig. 27), at others filamentous and branched, or they 

 form a layer or crust upon the objects to which they 

 are attached. The polypidoms, which are often some 

 inches in length, are frequently met with on the sea- 



M 3 



