Free-swimming Organisms Research Articles

Dinoflagellates and Their Use in Petroleum Geology: ABSTRACT

Dinoflagellates are a group of chiefly planktonic one-celled organisms abundant in modern seas and lakes. Their fossil organic shells range from 15 to 150 microns or more and are commonly found in Jurassic and younger marine sediments by the same techniques as those currently more widely used for spores and pollen. Commonly they occur in the same samples. Being surface-dwellers, dinoflagellates are relatively independent of the type of bottom sediment, although most abundantly found in marine shales; fresh-water fossil types are very rare. Rapid evolutionary changes, combined with wide geographic distribution of many species, make them excellent fossils for zonation and correlation. This is exemplified by successions of distinctive assemblages in the late Mesozoic and Cen zoic of Australia, by two assemblages containing many identical species in the Upper Jurassic of Utah and France, and by the worldwide distribution of a particularly distinctive form in the Upper Cretaceous. The value of dinoflagellates for environmental interpretation is as yet largely unexplored. Two major types of dinoflagellate fossils occur. One is the resistant shell, or theca, of the free-swimming organisms. This is commonly divided into polygonal plates and may contain a thick-walled and much ornamented protective structure, the cyst. The second type consists of isolated cysts, freed of their surrounding thecae. The latter type includes many of the minute spiny objects that have been called hystrichospheres. In fact, the majority of (but by no means all) post-Paleozoic hystrichospheres appear to be dinoflagellate cysts. Important criteria for distinguishing dinoflagellate genera and species include: the over-all shape, the number and arrangement of plates or of spine-like projections, the type of cyst, and the character of a special opening, the archeopyle, by which the rotoplasm left the theca or cyst. The literature on fossil dinoflagellates and the number of described genera and species are still small, but now explosively expanding as interest in the group increases. Although fossil dinoflagellates are already useful tools of the applied paleontologist, our understanding of them and the full development of their potentialities for applied paleontology are in early stages. End_of_Article - Last_Page 266------------

"Bioconvection Patterns" in Cultures of Free-Swimming Organisms

The moving polygonal patterns in dense cultures of Tetrahymena and other ciliates and flagellates look like "Benard cells," but are not due to thermal convection. They seem to be due to a similar dynamic instability that occurs when the energy input is internal and mechanical. The high concentration in the patterns may be useful in fertilization.

The Radiation Field near a Flat Applicator

On reading Dr. Robertson's paper it seemed that physical measurements of the radiation received by the organism might be of value in correlating the results with those of other observers, and we therefore undertook to make such measurements. As already stated, the applicator consisted of two silver discs between which were held 400 mgm. of radium in a circular area of 3·5 cm. radius. The two hundred radium cells were unfortunately not fixed in position, but, as will be seen later, little movement was likely under normal conditions as they could not pass over each other, and were fairly tightly packed. Photographic films placed in contact with the applicator when in (a) horizontal and (b) a vertical plane showed that little movement of radium took place on holding the disc vertical, probably not enough to alter appreciably the average value of the dose received by free swimming organisms at the distance at which they were normally irradiated in these experiments. The depth of liquid in which the protozoa were moving was approximately 5 mm., the mean plane of the liquid being at nearly 6·5 mm. from the mean active plane of the radium. Measurements were made with the system already described in this Journal,1 the normal ionisation chamber being replaced by a smaller cylindrical one, 5 mm. deep. The centre of the chamber was 6·5 mm. from the plane of the radium, the chamber therefore corresponding to the position of the medium inhabited by the organisms.