444 MANUAL OF THE MOLLUSCA. 



shallow pan, is the least convenient to keep ; and thei efore a large glass jar 

 is usually adopted, or an oblong tank, made to fit the recess of a window, 

 with slate ends and bottom, and plate- glass sides. 



The most convenient form of tank, is that recommended by Jlr. R. War- 

 ington ; it is a four-sided vessel, having the back gradually sloping upwards 

 from the bottom at an angle of 45 or 50 degrees, and the consequently ex- 

 tended top sloping slightly downwards and resting ou the upper part of the 

 back. The bottom is narrow, and the back may be covered with light roek- 

 work, extending just above the water-line, to afford places of growth to the 

 sea-weeds and fixed animals, and provide the litoral shell-fish with a feeding- 

 ground close to the surface. The front and top of this aquarium are of 

 glass, the rest of slate, fixed in a stout frame-work. (An. Nat. Hist. 14, 

 p. 373.) 



The aquarium should be covered, at least in towns, with a lid, or plate 

 of glass, to check evaporation and exclude dust. If ventilation is necessary, 

 the lid may be supported by small bent pieces of lead, hung on the rim of 

 the tank. 



The " balance of organic nature " is maintained in these aquaria by grow- 

 ing plants with the animals (p. 31, note). For fresh- water tanks, Fa/isfieria 

 spiralis is the best plant ; but if there is space for the common flag (in> 

 pseudaconis) or water-plantain {Alisma) they will rise above the surface and 

 blossom.* The Anacharis ahinastnim and Hi/drocharis (like a miniature 

 water-lily), may be grown at the surface. And if the tank is covered with a 

 frame filling the window, some climbing plants may be trained in it, and the 

 sides converted into a rockwork on which many ferns will thrive and expand 

 their fronds in the moist air. {TFamigton). 



For marine aquaria the green-weeds {JJlva, Entcromorpha and Bryopsis) 

 are better oxygen -producers than the red sea-weeds, but the latter are so 

 attractive as to be often tried.f The wxed may become too luxuriant and 

 require to be thinned in summer, but in the winter it dies down, and nearly 

 disappears. Some of the threadlike weeds (diatoraaceous algte) are apt to gain 

 admission, and in autumn break up spontaneously, filling the water with an 

 opaque green eh ud. 



The surface of rockwork, in the aquarium is liable to be overspread, and 

 the interior of the glass itself rendered opaque, by the early growth of con- 

 fervce. This may be in some degree prevented by keeping the water free 

 fi'om the grown plants, which are easily removed ; and the green on the glass is 

 kept in check by water-snails and periwinkles. These creatures occupy them- 



* When small 'i lies are kept in an aquarium, however limited, in which the 

 aquatic plants are grouped in the centre, they will swim round and round it in a 

 little shoi, 



t Mr. Warington recommends the employment of glass tinted green for mode- 

 rating the light when red sea-weeds are grown. 



