tliera having notliing wliatcver to do Avith the separation of
the cells, and never being in any case the result of it, but
depending, as explained already, upon unequal internal pressure,
and occurring chiefly at the period of reproduction.
The effect of the eversión is, as already observed, to occasion
tlie dislocation of the cells.
“ Again, in every filament of those Zygnemata which exhibit
the inverted structure, cells may be observed terminating
iu the ordinary manner of Confervoe, viz. by plane surfaces,
the presence or absence of the inversion depending upon the
Iicriod of the formation of the dissepiments ; the older ones,
or, as observed in the beginning of this notice, the more
mature ones only presenting it. Thus it folloAvs that the
opposed extremities of cells always exhibit the same structure,
and that this alternation in form supphes evidence the most
conclusive of the multiplication of cells throughout the entire
filament of a Conferva by division.” *
I t is in this genus tha t the radiated organ desci’ibed in the
Introduction is best developed, and may be observed with
most advantage ; it is here also that the cruciform organs,
supposed to secrete the raphides, and the curved strings of
cytoblasts, before noticed, are most clearly seen.
Attention to the structure of the cells in the Zygnemata is
of the very first importance in the discrimination of species,
it being th e only distinguishing character in some closely
allied species.
F irst Subgenus.— Filaments conjugating.
* Extremities o f the cells truncated.
a. Spires numerous.
1. Zygnema orbiculaee Hass.
Plate X IX . Figs. 1, 2.
Char. Filaments highly mucous, and o f a light green colour.
From a paper entitled “ Observations on some Points in the Anatomy
and Physiology of the Freshwater Algæ,” by Arthur H. Hassall,
inserted in the “ Annals and Mag. of Natural History,” vol. xii. pp. 26 8.
their diameter and length being very considerable. Cells,
lohen in a state o f conjugation, a little longer than hroad,
prior to which, however, they are frequently not h a lf so
long as broad; winding round the interior o f these arc
about eight spiral threads, the granules in them being small.
Sporangia almost circular, fattened.
Z. maximum Hassall, in Annals of Nat. Hist. vol. x. p. 36.
Hab. Cheshunt, Hertfordshire, and the country adjacent;
Netting H i l l : A. II. H. Waldron and W o rth in g : Mr.
Jenner. Graham Castle: Major Martin.
This is one of the finest as well as largest of all the Zygnemata
hitherto described, the diameter of the filaments greatly
exceeding those of Z. nitidum, the Conjugata princeps of
Vaucher. I t is found only in ponds and dykes whose waters
are deep and permanent, and it does not conjugate until near
the end of the summer. The only other Zygnema w ith which
it could possibly be confounded is Z . serratum, between which
and the present plant several well marked differences exist,
as will presently be shown. T avo varieties of this plant were
described by me in the 11th vol. of “ Annals and Magazine
of Natural History,” p. 432. both of which I noAV feel assured
are quite distinct. The first of these . Is Z . alternatum.
2. Zygnema alternatum Hass.
P la te X X .
Char. Filaments o f the same diameter as those o f the preceding
species. Cells rather longer than broad. Spires numerous,
granules small, conjugation alternate, that is, every second
pair o f cells alone conjoin.
Hassall, Annals of Nat. Hist. vol. xi. p. 432. Zygnema
Deeaisne, in Mémoire sur la Classification des Algues,
An. des Scien. Nat.
Hab. Cheshunt, Hertfordshire. A. H .H .
This is one of the rarest species of the genus, and I am not
sure that I have ever met with it more than once. The mode
of conjugation of the cells, the alternate ones only uniting, is