SMALL MARINE TANKS. 141 



they are about to moult, and such caverns and grottos 

 are to them very necessary. Very fine, well washed 

 sea or river sand should be strewn on the bottom of 

 the marine tank, and the depth ought to be adjusted 

 to the creatures it is intended to keep. If marine 

 worms, such as Sabella and Terebella, and even some 

 Crustacea, it should be of considerable thickness; but 

 always with fine shingle mixed with it, for these 

 creatures love to burrow in it. Shrimps take great 

 pleasure in dusting themselves with the fine sand, 

 descending into it and throwing it about them much 

 after the manner of birds. 



As only few kinds of sea-weeds can be successfully 

 grown in small aquaria, it follows that aeration or 

 oxydization of the water has to be compassed by 

 artificial means. The sea-lettuce ( Ulva^ latissima), 

 Fig. 100, is believed to give off more oxygen than any 

 other marine plant, and is well acted upon by the sun 

 in clear, shallow water, insomuch that we may often 

 see its bright green surface silvered over with minute 

 bubbles of oxygen. The best way to transfer sea- 

 weeds is to find small specimens rooted to pebbles or 

 rock, and bring them as they are thus found, taking 

 care not to break or bruise them, otherwise they will 

 slough. Cladophora is a sea-weed which grows readily, 

 and when its thread-like fronds are expanded, it 

 has a very pretty appearance. If the student is 

 not perplexed by a little extra trouble, there will 

 be little difficulty in growing other sea-weeds ; 



