222 BULLETIN OF THE NATURAL HISTORY SOCIETY. 
f 
with a somewhat rapid motion (though this is in reality magni- 
fied as well as the objects themselves) encountering or retiring 
from obstacles, and with every appearance of spontaneity. 
How it is done is* a puzzle not yet satisfactorily settled, but is 
doubtless connected with the internal activities of the cell and 
the work of that wonderful substance, Protoplasm- It was 
this property, no doubt, which led the earlier observers to regard 
the Diatoms as animals and to rank them with the group of the 
Infusoria as they are still often called. But no trace of organs 
of locomotion, such as are found in the latter, have yet been 
observed even with the most powerful microscopes. 
In order to get an idea of the structure of the Diatom cell 
it is well to compare the latter with one or two familiar objects. 
The first comparison is with an ordinary pill-box. This consists 
of a circular collar, with a fixed base and a moveable cover. 
If the base be also made moveable the resemblance to a Diatom, 
especially one of the circular forms (such as Coscinodiscus, 
Fig 15) is very marked. The Diatom also has two covers, 
termed “ valves,” and between these a collar or “ connecting 
membrane,” all easily separating from each other, and it is 
from this easy separation that the name of Diatom (dividing 
into two) is derived. Similarly, when compared with a watch, 
the valves correspond to the two lids of the latter while the 
“ conmecting membrane ” finds it counterpart in the circular band 
upon which the lids are fastened. In this case the comparison 
may be carried further, for the sculpturing on the valves of the 
Diatom correspond to the artistic designs with which the watch 
lids are usually ornamented, and in the case of certain circular 
forms the resemblance to an ordinary engine-turned watch is 
very striking. Even when the forms are not circular, the 
structure is essentially the same. All Diatoms, whatever their 
shape, consist, so far as the cell wall is concerned, of three parts, 
the two “ valves ” and the “ connecting membrane.” It is well 
to add that when viewing a Diatom the part turned towards 
the observer, if it be that of a valve, is regarded as constituting 
the “ lateral ” or “ valvular ” view, while if both valves are 
