I next exposed a quantity of Radiolarians (chiefly _Collozoum_) to sunshine, and was delighted to find them soon studded with tiny gas-bubbles. Though it was not possible to obtain enough for a quantitative analysis, I was able to satisfy myself that the gas was not absorbed by caustic potash, but was partly taken up by pyrogallic acid, that is to say, that little or no carbonic acid was present, but that a fair amount of oxygen was present, diluted of course by nitrogen. The exposure of a shoal of the beautiful blue pelagic Siphonophore, _Velella_, for a few hours, enabled me to collect a large quantity of gas, which yielded from 24 to 25 per cent. of oxygen, that subsequently squeezed out from the interior of the chambered cartilaginous float, giving only 5 per cent. But the most startling result was obtained by the exposure of the common _Anthea cereus_, which yielded great quantities of gas containing on an average from 32 to 38 per cent. of oxygen.
At first sight it might seem impossible to reconcile this copious evolution of oxygen with the completely negative results obtained from the same animal by so careful an experimenter as Krukenberg, yet the difficulty is more apparent than real. After considerable difficulty I was able to obtain a large and beautiful specimen of _Anthea cereus_, var. _smaragdina_, which is a far more beautiful green than that with which I had been before operating--the dingy brownish-olive variety, _plumosa_. The former owes its color to a green pigment diffused chiefly through the ectoderm, but has comparatively few algæ in its endoderm; while in the latter the pigment is present in much smaller quantity; but the endoderm cells are crowded by algæ. An ordinary specimen of _plumosa_ was also taken, and the two were placed in similar vessels side by side, and exposed to full sunshine; by afternoon the specimen of _plumosa_ had yielded gas enough for an analysis, while the larger and finer _smaragdina_ had scarcely produced a bubble. Two varieties of _Ceriactis aurantiaca_, one with, the other without, yellow cells, were next exposed, with a precisely similar result. The complete dependence of the evolution of oxygen upon the presence of algæ, and its complete independence of the pigment proper to the animal, were still further demonstrated by exposing as many as possible of those anemones known to contain yellow cells (_Aiptasia chamæleon, Helianthus troglodytes_, etc.) side by side with a large number of forms from which these are absent (_Actinia mesembryanthemum, Sagastia parasitica, Cerianthus_, etc.). The former never failed to yield abundant gas rich in oxygen, while in the latter series not a single bubble ever appeared.