The Aquarium. 5701 



VEGETATION. 



The plants best fitted for fresh water are the Vallisneria spiralis, 

 the Myriophyllum, Ceratophyllum, and the Anacharis, all of them 

 submersed plants, and fulfilling the purposes required most admirably. 

 From the great supply of food in the aquarium, the growth of the 

 Vallisneria is very rapid, and it requires, therefore, to be thinned by 

 weeding; this should never be done until late in the spring, and on 

 no account in the autumn, as it leaves the tank with a weakened ve- 

 getation at the very time that its healthy functions are most required. 



The vegetation of the ocean is of a totally different character and 

 composition, being very rich in nitrogenous constituents. There are 

 three distinct coloured growths, — the brown or olive, the green and 

 the red. For the purposes of the aquarium, where shallow-water 

 subjects are to be kept, the best variety is the green, as the Ulvae, the 

 Enteromorpha, Vaucheriae, Cladophora, &c. These should be in a 

 healthy state, and attached to rock or shingle, when introduced. We 

 shall have occasion to notice the Rhodosperms under the head of 

 < Light.' 



SCAVENGERS. 



A most important element in establishing and maintaining the per- 

 manent balance between the animal and vegetable life, without which 

 no healthy functions can be secured, and the aquarium must become 

 a continued source of trouble, annoyance and expense. The mollusk 

 which was first employed, the Limnea stagnalis, was found to be so 

 voracious as it increased in size that it had to be replaced by smaller 

 varieties of Limneae, by Planorbis, and other species of fresh-water 

 snail. The number of these should be adjusted to the quantity of 

 work they are required to perform. In the marine aquarium the com- 

 mon periwinkle fulfils the required duties most efficiently, and is 

 generally pretty active in his movements. The varieties of Trochus 

 are also most admirable scavengers; but it must be borne in mind 

 that they are accustomed to mild temperatures, and will not live long 

 in a tank liable to much exposure to cold. The Nassa reticulata not 

 only feeds on the decaying matters exposed on the surface of the 

 rock-work and shingle, but burrows below the sand and pebbles, with 

 the long proboscis erected in a vertical position, like the trunk of the 

 elephant when crossing a river. But in the ocean there are innume- 

 rable scavengers of a totally different class, as the Annelids, Chitons, 

 starfish, nudibranch mollusks, &c; thus affording a most beautiful 



