124 SEA- WATER AQUARIA. 



decay, that it is better to avoid them altogether, and to depend 

 only upon that which gradually and naturally appears upon the 

 rocks of the aquarium by the action of light, and which answers 

 every chemical purpose ; and the amount of growth of such 

 vegetation can always be precisely regulated by the amount of 

 light given to them — and so vigorously does it grow on places of 

 its own choice, that its tendency is to increase too much when the 

 light cannot be so far diminished without making an aquarium 

 too dark for objects to be distinctly seen in it — and in such cases 

 some animals, as the moUusk, the ormer {Haliofis), and the fish, 

 the grey mullet {Mugil), are employed to eat it down to the very 

 small quantity required to decompose the carbonic acid gas of 

 even the largest aquarium." 



In my tank there are two Periwinkles {Litlorina littorea), one 

 Top (Troc/ins umbilicatus)^ three Chitons (^Chiton fasciculaiis)^ and 

 one Dog Whelk {Pui-pura lapillus) ; and all these are at work 

 keejjing down the vegetation. The Trochus seems happy enough 

 and very busy ; so it is interesting to note, on Lloyd's authority, 

 that " the Top is of delicate organisation, and usually dies (in a 

 streamless aquarium) instead of doing any work." It is curious 

 to see how clean the Chitons sweep the rock-work. The bare, 

 raw-looking patches on the rock mark their track, and contrast 

 strangely with the brown or. green vegetation on either side. The 

 Periwinkles work very spasmodically, sometimes adhering to the 

 slate, close together, high out of the water for several days with- 

 out moving ; whereas at other times they are very active, clearing 

 off the vegetation and following it down into the porosity of the 

 rock-work, so as to leave little depressions or concavities. As to 

 the Whelk, he progresses but very slowly ; he keeps to a piece of 

 rock-work in a corner of the tank, and either works very little, or 

 does his business with great thoroughness, so long does he take 

 to get over a small space. I notice that this creature sticks to the 

 rock-work with great tenacity. 



I have between thirty and forty other specimens, some of 

 which are very fine and large, in my tank, and hope after a time 

 to procure more. Two of these specimens I may briefly allude 

 to, on account of the difficulty some have in keeping them alive 

 in captivity. One is Tealia Crassicornis^ the Dahlia Wartlet or 



