16 POPULAR HISTORY OF THE AQUARIUM. 



bivalve shells, such as the fresh-Avater Mussels, or marine 

 Oysters and Scallops, lie very quietly in their burrows, or 

 hang attached by their byssal cords, agitating the water that 

 surrounds them, and filtering it as it passes through from 

 the oral to the anal opening of the tubes ; thus exercising 

 a cleansing influence on the surrounding fluid. Gastero- 

 poda, or Crawling Molluscs, either with or without shells, 

 give variety and animation to a tank ; while Periwinkles 

 in salt-water, and ATater-snails in fresh, are useful as sca- 

 vengers, eating away the vegetable crust which obscures the 

 glass ; other Molluscs, such as 'Purpura, Buccinum, and 

 Nudibranchs (Plate XIY.), feed on animals, and must not 

 be put in the way of any choice living morsels which you 

 may wish to preserve. Cephalopodous Mollusca, or the 

 Cuttle-fish tribe, are too oceanic to live long in confinement. 



Folyzoa and Rotifera are of microscopic interest. 



Insecta,—Ti\^ Water Beetle (Plate XIX.) and Water 

 Spider (Plate XVIII.) are very interesting in their habits, 

 as described hereafter, and seem to live as well in the tank 

 or jar as they would in their natural localities at freedom. 



Crustacea are among the most amusing objects in a 

 marine collection. Many of them are useful in picking up 

 scraps of decaying animal-matter from between the stones. 



