1857.] on the Aquarium. 407 



We shall have occasion to notice the rhodosperms under the head of 

 light. 



Scavengers, — A most important element in establishing and 

 maintaining the permanent balance between the animal and vege- 

 table life ; without which no healthy functions can be secured, and 

 the aquarium must become a continued source of trouble, annoyance, 

 and expense. The mollusc 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 freshwater snail. The number of these should 

 be adjusted to the quantity of work they are required to perform. 

 In the marine aquarium, the commqp perri winkle fulfils the required 

 duties most efficiently, and is generally pretty active in his move- 

 ments. The varieties of trochus are also most admirable scaven- 

 gers ; 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 rockwork 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 

 innumerable scavengers of a totally differing class, as the annelids, 

 chitons, starfish, nudibranch molluscs, &c. ; thus affording a most 

 beautiful provision for the removal of decaying animal matter, and 

 converting it into food for both fish and man. 



Light. — It is most probable that the greater amount of failures 

 with the aquarium have arisen from the want of a proper adjust- 

 ment of this most important agent ; the tendency being generally to 

 afford as much sun's light as possible ; but, on consideration, it will 

 be found that this is an erroneous impression. When the rays of 

 light strike the glassy surface of the water, the greater part of them 

 are reflected, and those which permeate are refracted and twisted 

 in various directions by the currents of the water ; and where the 

 depth is considerable it would be few rays which would penetrate 

 to the bottom : but let the surface become ruflfled by the passing 

 wind, and it is little light that can be transmitted ; and when this 

 same disturbing causes lashes into waves and foam, not a ray can 

 pass, and all below must be dark as night. Too much light should 

 therefore be avoided ; and the direct action of the sun prevented 

 by means of blinds, stipling, or the like. It is a great desideratum 

 to preserve the growth of the lovely red algae in all their natural 

 beauty, and prevent their becoming covered with a parasitic growth 

 of green or brown coloured plants ; this can be effected by modify- 

 ing the light which illuminates the aquarium by the intervention of a 

 blue medium, either of stained glass, of tinted varnish, coloured 

 blinds, &c. The tint should be that of the deep sea, a blue free 

 from pink, and having a tendency rather to a green hue. This 

 modified light affects also the health of those creatures which are 



