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This month I'll talk about

Placocista spinosaGreek: plax - plate; kiste - box

P lacocista spinosa is a filose amoeba, order Euglyphida, family Assulinidae.Filose amoebae have filiform pseudopodia, while the lobose amoebae of the

(not closely related) order Arcellinida have broader lobose pseudopods.

With a size of up to 145 µm it is the the largest of the Euglyphida. It is tyrphobiont,i. e. limited to wet acid [pH < 4.5] sphagnum bogs. Even there it is rare. Appearsworldwide in all climates.

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Animal with a compressedoval, hyaline, colorless

shell, with acute border and termi-nal elliptical mouth; the border ofthe latter entire. Shell composed oflongitudinal rows of alternatingoval or roundish plates overlap-ping at their contiguous borders,so as to produce hexahedral areaslimited by zones of minute ellipses.

Lateral borders and fundus fur-nished with acuminate spines ar-ticulated with the shell.

Sarcode and pseudopods as inEuglypha.

(Joseph Leidy, 1879)

Some papers mention symbioticalgae. Please see HERE.

My specimen had no chlorellainside.

aboral view

empty shell, oblique light

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Most of my shells where either emptyor in the encysted condition as the

one on the front page. Rarely I foundspecimen in a more active condition likethe one above. Leidy: "the mass of sar-code undergoes contraction from thesides of the shell, but appears to retain aclose connection with the fundus."

The large bright area near the fundus isthe nucleus with a central nucleolus.

The blob to the right is a contractilevacuole. Like in many single celled freshwater organisms this organelle serves asa protective mechanism that prevents thecell from absorbing too much water dueto osmosis - kind of kidney.

I have never observed emitted pseudo-pods.

Euglyphids produce the plates andneedles for the next generation with-

in their Golgi-apparatus and store theminside until ready for the next cell divi-sion. The bottom part of the specimen onthe left is packed with this reserve build-ing material.

Then, in an awesome process, they me-ticulously distribute the scales and nee-dles over the surface of the new test.

oblique light

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All comments to the author Hans Rothauscher are welcomed.

Lookalike

Euglypha strigosa is almost as large as Placocista spino-sa and at first glance very similar.

However, is has needles all over the shell, not only alongthe lateral borders, and about 10 large dents around itspseudostom.

Hans Rothauscher, July 2019

Further reading:●The Leidy Portal.●Ferry Siemensma.●My Page.

some of Leidys beautiful illustrations

Published in the August 2019 issue of Micscape magazine.www.micscape.org