36 MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES. 
“swarming.” It is difficult to imagine the kind of motion possessed by the 
protoplasm in which these swarming microsomata are embedded; but however 
closely it is confined, there must be continual rapid displacements in its substance, 
which is very fluid, and it may be assumed that here again it is not so much 
the tiny grains that bestir themselves as the protoplasm which holds them. 
Probably the protoplasmic matter spreads and stretches out and rotates, and 
individual granules are carried about by it. This, of course, does not exclude 
the possibility of the granules possessing a vibratory motion of their own within 
the mass of protoplasm. 
Similar, but not identical, is the swarming movement of protoplasm observed 
in cells of the Water-net (Hydrodictyon utrieulatwm), and in several other plants 
allied to it. Hydrodictyon looks like a net in the form of a sae, and composed 
of green threads. The meshes of this net, which are generally hexagonal, consist, 
however, not of filaments but of slender cylindrical cells joined together by threes 
at their extremities, somewhat in the same way as are the leaden frames of the 
little hexagonal panes of glass in gothic windows. The protoplasmic body of 
one of these cells in due time breaks up into a great multitude (7000-20,000) of 
tiny clots, which begin to move and swarm within the cell-cavity in what appears 
to be a disordered medley. In half an hour, however, the excited mass is again 
restored to rest: the minute particles take form and arrange themselves in definite 
order, each having two others at either extremity, making an angle of 120° with 
it; and, lastly, all unite to form a single tiny net having exactly the same shape 
as the one whose component cell constituted the arena of this process of construc- 
tion. The miniature water-net so formed then slips out of the cell, the latter 
opening for the purpose, and in from three to four weeks it grows to the same 
size as the parent plant. 
In the above we have an instance of a protoplast producing a whole colony 
of cells, which are obliged to leave their home for want of space. In cases 
previously considered we have found the protoplast stretching and elongating 
in all directions, drawing itself out into bridles and spreading as a delicate lining 
to walls, and so endeavouring generally to expand and present the greatest surface 
possible. Again, we have seen it wandering freely, creeping, swimming, and 
rotating, and by this method also covering as much space as it can. But, con- 
versely, there is a time when a protoplast tends to the other extreme; the 
expanded mass of its body gathers itself together again, contracts more and 
more, and at length becomes a resting sphere, that is to say, it assumes the con- 
figuration which exposes the least surface to the environment. 
This process exhibits itself with particular clearness within the cell-cavities 
of the green alge known by the name of Spirogyra, a species of which is 
represented, magnified three hundred times, in Plate I, fig. . In this alga 
the protoplasm in each mature cell-cavity forms, as a general rule, a very deli- 
cate parietal lining wherein green chlorophyll bodies are embedded, arranged 
in a spiral band. All of a sudden, however, this lining strips itself off the inner 
