264 A Brief History of a Marine Tank. 



During the first winter all the tufts of Ulva and Enteromorpha 

 perished. I thought then, and I still think, that in winter 

 such a tank ought to be turned round to face the light, and be 

 turned back to its former position some time at the end of 

 February or the beginning of March. I wish I had done so, 

 it would have made the experiment more complete probably. 

 However, I . did not ; the fact is, such things are not easily 

 moved, and so, if it occurs to one's mind to move them, the 

 intention is scarcely likely to be carried into effect. Before the 

 first winter set in, there was a perceptible green growth on 

 some of the shells and stones, but not till the summer of 1863 

 was the growth of vegetation at all general. Then I became 

 aware of the immense value of the old shells; for the old, 

 rough, much pierced, and sponge- covered oyster shells acquired 

 a rich deep green hue, and equally well clothed were the 

 shells of the barnacles. On these latter, indeed, vegetation 

 first appeared, and they are, no doubt, invaluable if well 

 cleansed of animal matter before being inserted. But from 

 first to last there has never occurred a single instance of 

 "pea-soupiness," no ropy confervee have been produced. 

 The vegetation amounts to no more than a velvet-like, deep 

 green coating on shells and stones, and this is richest on bar- 

 nacles and oyster shells. 



In every case of death subsequent to those particularized 

 above, I have allowed the water to right itself, which it does 

 in the course of a few days, or a week, the cloud spreading at 

 first all over the tank, and then contracting to a black spot, 

 which marks where the animal perished, and in the course of a 

 fortnight (or less) this entirely disappears. It is an important 

 fact that when the animals become seasoned, and the whole 

 affair is left alone, one death does not necessarily lead to another, 

 as in a tank heavily stocked, and exposed to a strong light ; 

 but as soon as the cloud caused by decomposition begins to 

 spread, all the creatures close their disks, the mollusks con- 

 tract and draw down their operculi, and in this state of torpor 

 they all remain till things improve, when they again resume 

 their activities. The most frequent cause of impurity is the 

 result of feeding ; for scraps of oyster or mussel given them 

 are sometimes immediately ejected, and fall among the shells, 

 and remain hidden, to cause putrefaction. The only means of 

 preventing this is to remove every particle of food not received 

 by the animals ; to feed seldom, so as to increase the proba- 

 bility that food will be welcome ; and, lastly, to feed only on 

 bright mornings, when the water is nearly transparent. Once 

 a fortnight is often enough to feed all the summer, and once in 

 five or six weeks during winter. 



During the first few months after the tank was fitted, it 



