26 THE AQUARIAN NATURALIST. 



the habits of its inmates. The want of light, on the 

 other hand, causes the rapid decay of the vegetation, 

 and the products arising from this change are highly 

 poisonous to animal life, the whole contents of the 

 aquarium becoming of a black colour, and very soon 

 of an offensive odour. 



All marine productions dislodged from considerable 

 depths are liable to injury. Though casually obtained 

 clean and entire, most of them are profusely invested 

 with parasites, which, fatally and invisibly wounded, 

 speedily corrupt the circumambient fluid in their 

 decay. The water also sometimes contracts a noxious 

 principle from causes eluding conjecture and obser- 

 vation, and it may be fit to warn the inexperienced 

 naturalist against employing tall vessels with much 

 empty space above the water on all occasions ; for if 

 the air vitiated by respiration, after escaping from 

 the surface, be reabsorbed, it cannot be otherwise 

 than deleterious. This is well illustrated by keeping 

 fishes in deep vessels with a scanty proportion of 

 water, to prevent their leaping over the side; they 

 generally perish in a short time, and hence shallower 

 vessels nearly full are always preferable. 



It is sometimes advisable to suspend delicate or 

 interesting zoophytes, more especially of the Tubu- 

 larian or Sertularian races, by means of silken threads 

 attached to pieces of cork, which may be allowed to 

 float upon the surface of the tank. This is a method 

 often to be particularly recommended, both for con- 

 venience and security : the subject is better preserved ; 

 the specimens are more accessible, should they be re- 

 quired for microscopic examination, and by immediate 



