. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . e state. The division is usually longitudinal, although ina few cases, such as in Gymnodinium rufescens, it is transverse (fig. 38 B).Unfavourable conditions of existence rapidly bring about a quiescent, en-cysted state, in which multiplication may sometimes occur. In this non-motile state the cell may be surrounded by a thick gelatinous envelope(as in Gymnodinium Zachariasi) or only by a thin membrane (as in Hemidiniumspp. and Gymnodinium fu
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . e state. The division is usually longitudinal, although ina few cases, such as in Gymnodinium rufescens, it is transverse (fig. 38 B).Unfavourable conditions of existence rapidly bring about a quiescent, en-cysted state, in which multiplication may sometimes occur. In this non-motile state the cell may be surrounded by a thick gelatinous envelope(as in Gymnodinium Zachariasi) or only by a thin membrane (as in Hemidiniumspp. and Gymnodinium fucorum). After the division of the parent-cell thetwo daughter-cells are set free either by dissolution of the gelatinousenvelope or by the bursting of the thin membrane. A resting-state withthick cellulose walls has occasionally been observed in some forms. 1 E. Chattou (Complex Rendun Aead. Sc. Paris, cxliii, 1906; ibid, cxliv, 1907) has alsodescribed several parasitic species of the genus lllastodinium, which he regards as the type ofa new family of the Peridinieas. These organisms are parasites upon various pelagic Copepodsand 54 PeridiniesR ~, Most of the Gymnodiniacese are marine organisms, although many speciesof the genus Gymnodinium occur in fresh waters, generally in small poolsand ditches containing much vegetation. Spirodinium, Cochlodinium, andothers are exclusively marine, but the species of Hemidinium chiefly inhabitfresh water. Herdman (11) has furnished some interesting observations onthe occurrence of Amphidinium operculatmn Clap. & Lachm. on the beachat Port Erin in the Isle of Man. It is evidently a shore-form, living on thesurface of the sand, and like other Peridinians, sensitive to light. Herdmansobservations tend to show that it is either periodic or spasmodic in itsappearance, but insufficient is known concerning the physiological require-ments of these organisms to enable any definite statements to be maderegarding their
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