YOUNG OF OYSTERS. 241 



transparent. Mr. Lloyd believes this is due to their 

 breathing out quantities of carbonic acid gas, and thus 

 converting the carbonate of lime which gave the water 

 a milky appearance, into a ^-carbonate, when the 

 visible carbonate is taken into suspension and thus 

 rendered invisible. Oysters are therefore to be seen 

 distributed in nearly all the Brighton tanks, on ac- 

 count of the temporary service they render in keeping 

 the water transparent. Their semi-opened shells 

 exhibit the plates of gills (commonly called the 

 " beard " of the oyster). These gills are richly clothed 

 with active cilia, which create currents in the water, 

 and thus the gills are constantly bathed by fresh sup- 

 plies of oxygen, the same currents also bringing food. 



Fig. 172. 



Young Oysters, with their natatory or swimming gills. 



A good deal of the turbidity of sea water in tanks 

 is often due to the presence of myriads of the 

 zoospores of algae. These zoospores have a wrig- 

 gling motion, so that few people uninstructed in 

 the life history of sea-weeds, would imagine them to 

 be connected with the reproductive parts of maritime 



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