and Foraminifera. The relationships of these groups (and all eukaryotes) are under
review, based on DNA sequences and possession (or not) of particular proteins. There
has been a tendency to create new names for possible evolutionary lines (new
kingdoms, supergroups, phyla) with every shift in the phylogenetic reconstructions.
Agreement regarding phylogeny will eventually emerge when enough sequences from
enough genes (and amino-acid sequences in proteins, readable from DNA, to reduce
the effects of code synonymies), and from enough distinct groups, are included in the
analysis. At present, Radiolaria and Foraminifera appear to be one clade (i.e. related
on an evolutionary branch) recently named “Retaria” (almost certain to change); the
others are more distantly related to them and to each other (Moreira et al. 2007). It is
fair to suggest that pseudopods evolved very early, possibly more than once (there are
several distinct types), and are retained by numerous, now distantly related, groups.
(^) Cells of the Acantharia are constructed around radiating spicules of crystalline
strontium sulfate (celestite), which dissolves rapidly after cell death. They are
probably the most abundant protozoans with hard skeletons. Heliozoa and Radiolaria
(“rads”), although not necessarily closely related, have a siliceous central capsule
within the cytoplasm surrounding the nucleus and several other organelles; the outer
cytoplasm is partly occupied by food vacuoles. Radiolarian skeletons are quite
durable, and although small they sink with particulate detritus to form sediments
called radiolarian ooze. Polycystine rads have a billion-year fossil record; their
position in phylogenetic analyses suggests that most of the rhizopod groups were
distinct long before the Cambrian period. Most Phaeodarians have proteinaceous or
chitinous capsules, in some cases incorporating amorphous silica, bearing elaborately
branched spines. A few, like Phaeodina, are naked. A brownish mass near the nucleus
is termed a phaeodium and believed to be accumulated food waste. Ernst Haeckel
produced some wondrously elaborate pictures of Phaeodaria in the Challenger
Reports, together with a complex taxonomic system. He was accused of taking
excessive artistic license with his drawings, but eventually his observations have been
confirmed. “Forams” have larger, calcareous capsules surrounding their nuclei that
sediment as foraminiferan ooze. There are not only planktonic forams, but
morphologically distinct benthic forms. Together with radiolarian deposits, these
sediments provide us with valuable stratigraphic records. In many cases, distinctions
in isotopic composition between benthic and planktonic forams in the same buried
sediments are informative about past hydrographic differences between deep and
shallow ocean layers.
(^) In all of the shelled Rhizopoda, the pseudopods slide in and out along a spray of
fine spines extending from the cell center. They capture particulate food with those
pseudopods, ingesting it by phagocytosis. There is a second layer of cytoplasm around
the shell and base of the spines. In forams and rads this layer is highly vacuolated
(Plate 6.1) and may provide buoyancy by favoring lighter ions internally (e.g.