INTEODÜCTION.
The Desmids, to which this volume is devoted, constitute
the first family of the Zygophyceæ,* or, according to some,
form a distinct section of the Conjugatæ, closely allied to the
Zygnemaceæ.
The configuration of the cells is multiform, from the single
cylindrical and spindle-shaped, to those with most complicate but
symmetrical, lobate, or dentate outlines, whilst each species
retains always its characteristic form. In most cases each cell
is divided into two equal and symmetrical halves by a cross
constriction, which is either a shallow groove or. a deep and
narrow incision, so deep as to convey to some observers the impression
that each cell consisted of a pair of closely united
bodies of equal dimensions. These cells live separate from each
other, and are free, or else are united end to end in a filament,
whilst each cell retains its own individuality. They are surrounded
by a cellulose membrane, which, in some cases, must
contain considerable inorganic matter, since, when calcined
carefully, the remains still retain their original form. As
Ralfs f long ago intimated, “ all the Desmidieæ are gelatinous.
In some, the mucus is condensed into a distinct and well-defined
hyaline sheath or covering, as in Desmidium cylindricum and
Staurastrum iumidum; in others it is more attenuated, and the
fact that it forms a covering is discerned only by its preventing
the contact of the coloured cells. In general, its quantity is
merely sufficient to hold the fronds together in a kind of filmy
* See Cooke, “ Fresh Water Algæ,” p. 74.
t Ralfs, “ British Desmidieæ,” p. 15.