iO GENERAL HISTORY OF THE INFUSOEIA. 



ment causes these creatures to disgorge their food ; and if, after an interval, 

 the water be carefully poured off, a sediment will be found at the bottom, 

 which will consist of Diatoms mixed with some refuse." 



Dr. Donkin lately (T. M. S. 1858, p. 11) called attention to the occiuTence 

 of that rare form, Syndendrimn diadema, in the stomach of the lobster, and 

 in a subsequent paper {op. cit. p. 14) alludes to the abundant deposit of Uving 

 Diatoms upon the sands at the sea-side, in the follomng paragraph : — 

 " Professor Smith states that ' the shallow pools left by the retiring tide at 

 the mouths of our larger rivers ' are the favourite habitat of marine species. 

 But such localities I have found not to be half so prolific in species as the 

 smids of stdl bays, or the sJiore, where they are exposed hy the reflux of the 

 tide, at a distance corresponding ivith the half-tide margin. In these places, 

 where the sands are sloping towards the sea, and grooved out into small 

 fuiTows, filled -with salt water oozing out from behind, the abundance of 

 Diatoms aggregated into a living mass imparts to the surface of the sand 

 difi'erent hues of chestnut and oHve, the difterence of colom- being due to 

 the natiu'e of the species present. These coloured patches, it is interesting 

 to observe, are, during the sunshine, studded with numerous minute air- 

 bubbles, undoubtedly given off by the Diatoms themselves. 



^' To separate the Diatoms thus detected, from the surface of the sand, I 

 found to be impossible. I therefore seized hold of the nearest bivalve shell 

 which happened to lie in the way, and with this I carefuUy scooped up the 

 surface of the coloured sand. This I emptied into a wide-mouthed, stoppered 

 bottle, capable of holding eight ounces, until half full ; the other half of the 

 bottle I filled up with salt water. I then shook the whole briskly and allowed 

 the bottle to stand for a short period. The sand, being composed entirely of 

 fine round grains of quartz and the minute fragments of shells, settled at the 

 bottom in a few seconds, leaving the Diatoms all suspended in the water 

 above, and forming by their abundance a chestnut-coloured cloud, but not 

 more than 1 part in 1000 of the whole sand collected. The coloured water 

 was then poured into another bottle, and formed the gathering, while the 

 sand was thrown away. The Diatoms, in theii' tiu^n, were separated from the 

 superfiuous water by subsidence, and brought home in l|-oz. bottles. In this 

 manner I soon found that any quantity could be collected in a pure and un- 

 mixed condition, affording an excellent opportunity of examining their living 

 forms, and one of which I availed myself on Qxevj occasion. 



*' After carefully exaroining materials collected in this way from various 

 parts of the beach, I -detected not less than about 100 species, all these 

 strictly marine, and, with a few exceptions, each species in considerable 

 abundance." 



The fact of Diatomeae rendeiing themselves perceptible to common vision 

 by their excessive accumulation and the coloiu" they impart to water, is illus- 

 trated by the phenomenon of coloration of the sea recorded by Dr. Hooker, 

 also by the Melosira ochracea, which occui's in many, perhaps in all, cha- 

 lybeate waters, and also in peat water containing a small proportion of iron. 

 It is of the colour of ii^on rust, and in mineral springs, in which it abounds, 

 is often taken for precipitated oxide of iron. It covers everything under 

 water, but forms so delicate and floccose a mass that the least motion dissi- 

 pates it. In the spring of the year this mass is composed of very delicate, 

 pale-yellow globules, which can be easily separated from each other. They 

 unite together in rows like short chains, and produce an irregular gelatinous 

 felt or floccose substance. About summer, or in autumn, they become de- 

 veloped into more evidently articulated and stiff threads of a somewhat larger 

 diameter, but still form a complicated mass or web, and, either from adhering 



