Atractomorpha echinata gen. et sp. nov. is described from isolates derived from zygotes present in a dry soil sample obtained from Texas. The new genus is distinguished from Sphaeroplea primarily by its pattern of vegetative growth. While Sphaeroplea is distinctly filamentous with numerous coenocytic cells uniseriately arranged, Atractomorpha grows as individual, multinucleate, spindle‐shaped cells with sharply pointed extremities. Such cells may vary considerably in length (25–6000 μm, or more) and normally lack septa. In young, rapidly growing cultures the cells often attain lengths of 300–500 μm, but rarely exceed 1800 μm. The new species is further characterized by: (1) the regular formation of biflagellate zoospores in asexual reproduction, (2)anisogamy (occasionally oogamy) and (3) the size and ornamentation of its zygotes. Variations in vegetative morphology are discussed as are conditions for obtaining gametogenesis.

The ultrastructure of Pyramimonas virginica, the only known ultraplanktonic species of Pyramimonas, has been examined in detail. This species differs from other species of Pyramimonas in the structure of the flagellar apparatus, the scaly periplasL the pyrenoid, the eyespot, location of the eyespot in the cell, and mitosis. The flagellar apparatus is simple and shows signs of reduction. A 2-2-2-2 microtubular root system is present but a rhizoplast was not found. A very short transitional helix (perhaps two rings) is present in the flagellar transition region, together with a short one-piece stellate structure. Numbering of individual fagella in different species of Pyramimonas has shown a surprising variation in the orientation of the flagellar apparatus relative to the rest of the cell. Thus the eyespot in the various species is located differently relative to the flagellar apparatus and to other organelles such as the nucleus (in Pyramimonas there is no connection between the flage[lar apparatus and the eyespot via a flagellar root). Considerable experimentation has apparently taken place with regard to the orientation of the photoreceptor apparatus during the evolutionary history of the prasinophytes. In more advanced green algae this variability does not occur. In Pyramimonas four different flagellar apparatus/eyespot orientations have been detected, which has led us to a modified classification of the genus at the subgeneric level. New subgenera are proposed for P. virginica (subgenus Hexactis) and P. longicauda (subgenus Macrura).

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Mitosis and cell division have been examined ultrastructurally in the vegetative cells of Hydrurus foetidus (Vill) Trev. and found to resemble that of Ochromonas in two important aspects. First, the rhizoplast acts as the spindle organizing body and second, the spindle elongates considerably during anaphase. It differs from Ochromonas in that there is no movement of the basal bodies and flagella towards the poles. Moreover, the nuclear envelope remains relatively intact throughout early stages of mitosis, with gaps developing at the poles during prophase to permit entry of spindle microtubules. Disruption of the nuclear envelope does not occur in the equatorial plane until late anaphase. The spindle persists into telophase and is bent towards the posterior of the cell by the ingrowing edge of the cleavage furrow. Persistence of the spindle and lack of Ochromoms‐type cell elongation may be related to the constricting presence of the sheath during cell division—a completely different strategy to that adopted by the green algae under conditions of similar constraint.

Icosahedral, virus-like particles (VLPs) have been studied in attached germlings of Cylindrocapsa geminella Wolle after the settling of zoospores produced by a heat-shock treatment. About 26 h after zoospore attachment, 3–10% of the resulting one-celled germlings contained a massive concentration of VLPs filling a region at the basal (i.e., attached) end of each cell. This includes the area normally occupied by the nucleus which is completely lysed by the time the VLPs are first observed. VLPs were found only in single-celled germlings, although the samples included multicellular germlings. Organelle degeneration occurs in the infected germlings and this apparently leads to cell lysis. Mature VLPs generally appear six-sided in section and have a maximum diameter of 200–230 nm. They possess a finely fibrillar core surrounded by a multilayered, membranous coat. The varied appearance of the virogenic stroma is discussed as well as putative stages in viral assembly. Cultures of Cylindrocapsa are being maintained from which VLP-infected germlings can be routinely obtained. The possibility that these cultures may be carrying a latent virus infection which can be induced by heat shock is discussed.