198 
F. EANSOM—DIATOMS : 
early in the eighteenth century, and the first undoubted illustration 
occurs in the * Philosophical Transactions of the "Royal Society ’ for 
1703. In 1745 William Arderon described what he terms the 
“oat-like animal” (evidently some Navicula , a genus of diatoms) 
associated with the “hair-like insect” as he designates the Oscilla- 
torias. The most evident reason for thus placing in the Animal 
Kingdom the representatives of these two families, which are now 
regarded as undoubted vegetables, was the power that both possess 
of individual movement apparently quite independent of any ex¬ 
terior agency. A more extended knowledge of plants has clearly 
demonstrated that motion of this description is not confined to 
animals, but is also frequent amongst the lower forms of vegetable 
life. At the beginning of this century much confusion existed, and 
the Naviculae were regarded as one stage in the life of the Oscilla- 
toriae. Shortly afterwards a remarkably sceptical observer (Ingen- 
housz) maintained that, in certain forms at least, their character¬ 
istics were not sufficiently defined to warrant their being placed 
either amongst animals or amongst plants, and he considered 
that their true position was in the Mineral Kingdom. Such degra¬ 
dation however was not lasting, and in 1816 a distinction was 
made, and some species were classed as animals, while others were 
regarded as plants. De Candolle and others correctly regarded the 
whole family as vegetables. About the year 1822, Bory de St. 
Yincent, being unable to satisfy himself in placing several of the 
lowest forms of life amongst vegetables or animals, created a sort of 
intermediate Organic Kingdom, which he called Arthrodiees, and 
in which many diatoms were included. In 1831 Ehrenberg re¬ 
marks that he had succeeded in establishing their place in the 
Animal Kingdom, and they are thus classed in his valuable work 
on the Infusoria. Here they appear to have remained undisturbed 
for some time, but gradually, as general biological knowledge in¬ 
creased, and as the microscopes at the disposal of observers became 
greatly improved, doubt began to arise, and in 1853 the Kev. 
William Smith, in his standard work entitled * A Synopsis of the 
British Diatomaceee,’ unhesitatingly classifies the whole family as 
a Natural Order in the Sub-class Algse of the Vegetable Kingdom. 
Here they still remain, and there seems no probability of any 
further change. The desmids have frequently been classed with 
diatoms, but the absence of any siliceous cuticle, besides other im¬ 
portant differences, has now caused them to be regarded as a 
separate order. Thus we find that during their comparatively short 
existence in the domain of science, diatoms have been allotted to 
each of the three kingdoms in turn, and have also for a short time 
lived in a fourth, which was brought into existence largely for 
their benefit. 
Having briefly reviewed the history of the Diatomacese, it is now 
necessary to enter more fully into their structure. Each cell, or 
frustule as it is termed, is either free, attached to one or more of its 
own species, fixed to some foreign body, or enclosed with many 
others in a gelatinous envelope. Each frustule is seen to possess 
