THE FRESHWATER AQUARIUM 43 



weeds, Utricularia and Nitella have often thriven for 

 years together. Duckweed and Riccia float at the surface. 

 Mollusks have been introduced intentionally, and small 

 animals of various kinds casually, together with water- 

 weeds. The egg-laying and development of the mollusks 

 has been a favourite study. Dragon-fly larvae, larvae of 

 more than one species of Chironomus, Triclads, Hydra, 

 Nais and a variety of infusorians are among the things 

 that have appeared as it were spontaneously. No fishes 

 have been kept in this aquarium, lest they should devour 

 things of greater interest. 



It is worth while to keep a freshwater aquarium, if 

 only to see how the different water-weeds behave during 

 winter. Many of them form special winter-buds, bright 

 green and closely wrapped, which lie at the bottom during 

 all the hard weather, but float to the surface in spring, 

 being then buoyed up by the formation of air-bubbles in 

 their cavities, and set free by the decay of the old stems. 

 Nothing is then required except a free flow of water to 

 disperse them widely. 



The student of live nature will often require small tanks 

 for special purposes. Things that he wants to keep his 

 eye upon would perhaps be devoured or lost in the big 

 aquarium. Rectangular vessels, narrow and deep, with a 

 broad surface exposed to the light, are very convenient 

 for the study of isolated organisms, and happily they are 

 not at all expensive. 



It is almost as easy to manage a marine aquarium as a 

 freshwater one. We keep in our laboratory at Leeds a 

 good-sized marine aquarium, as well as a bell- jar filled 

 with sea-water. Into one or other minute animals and 

 plants received from the Marine Biological Laboratory at 

 Plymouth are put when they have served their immediate 

 purpose, and abundant material for study is thus kept 

 ready to hand. Both tanks have kept perfectly whole- 

 some for several years without ever being emptied. 



