Order 8. =Valoniaceæ.= The thallus is generally multicellular, without differentation into stem- or leaf-structures, but the cells are sometimes united together and form a leaf-like reticulate expansion (_e.g. Anadyomene_). Zoospores are known in some, and they are then formed directly in the vegetative cells. In others (_e.g. Valonia_), a mass of protoplasm, which maybe separated through the damaging of a cell, can surround itself with a cell-wall, and grow into a new plant. No other modes of reproduction are known. The most important genera are: _Valonia_, _Siphonocladus_, _Chamædoris_, _Struvea_, _Microdictyon_, _Anadyomene_. They are all salt water forms. [Illustration: FIG. 59.--_Caulerpa prolifera_ (natural size).] As already pointed out, the _Valoniaceæ_ occupy a somewhat central position among the Siphoneæ, and present points of similarity and contrast with the _Botrydiaceæ_ and the _Bryopsidaceæ_ through _Valonia_, with the _Dasycladaceæ_ through _Chamædoris_, and also with the _Cladophoraceæ_ through _Siphonocladus_, and _Struvea_. Order 9. =Dasycladaceæ.= The thallus consists of an axile longitudinal cell, destitute of transverse walls, attached at the base by root-like organs of attachment, and producing acropetally whorls of united, single or branched, leaf-like structures with limited growth. Asexual reproduction is wanting. Sexual reproduction by conjugation of gametes which arise in separate, fertile leaves, either directly or from aplanospores, which develope into gametangia. The principal genera are: _Acetabularia_, _Dasycladus_, _Neomeris_, _Cymopolia_. All marine. [Illustration: FIG. 60.--_Halimeda opuntia._ Plant (natural size). _B_ Part of a longitudinal section.]