PROGRESS IN PROTOZOOLOGY

PHYLOGENETIC RELATIONSHIPS AMONG PROTOZOA 197
pore near the center. These and other structures regress at different
stages of development and in the degree of juvenility in phylogeny.
They are best seen in sporozoites, the product of the sexual phase, and
function particularly in the invasion of a host cell, which is an important
feature of parasitism.
Three factors are of importance in the phylogeny of APICOMPLE-
XA: 1. The Stage: characteristic organelles may be present at one
stage but absent in another (e.g., tubular mitochondria, cf. Vivier);
2. The Host: It is essential to consider the phylogeny of both vertebrate
and invertebrate hosts in heteroxenous examples, as this does
not necessarily run parallel with the phylogeny of the parasite — unfortunately
there are few indications as to the date when the parasite
became established; 3. Zoogeography. The presence or absence
of certain parasites in various vertebrate groups in different regions of
the world may be linked with major geological events such as continental
drift in more remote ages and glaciation in more recent. Factors
2 and 3 are not directly related to classification, but undoubtedly
shed light on phylogeny. The intervention of a biting arthropod
has long been suggested as the path taken in the phylogeny of
haemosporidia from coccidia and of certain haemogregarines from
adeleids.
According to Prof. G a r n h a m, the question of free-living ancestors
of APICOMPLEXA has assumed new importance in light of the recent
observations on Acanthamoeba and Naegleria, which have illustrated
how the host barrier in parasitism can be surmounted. However,
ideas about their origin remain highly speculative. We look for three
primary characters — a degree of "amoebicity," a flagellate stage, and
some indication of sexuality. The members of the suborder Bodonina
Hollande, 1952 emend. Vickerman, 1976 have been suggested as the
possible ancestors of APICOMPLEXA. However, Prof. Garnham's
preference is for RHIZOPODA von Siebold, 1875, some members of
which possess at least two of these characters, e.g., Naegleria in the order
SCHIZOPYRENIDA. The "monopodial" cylinder found in some members
of this group resembles the structure of the motile zygote (ookinete).
The best secondary clues are offered by the sexual stages, e.g., the
microgamete and the zygote, which certainly are of immense significance
in the more recent phylogeny. Prof. G a r n h a m was still uncertain
as to how much help will be obtained through the employment
of biochemical features, e.g., isoenzymes and nucleic acid series.
Dr. BEYER was the second discussant. She found no justification
for the doubts expressed by some workers with regard to the validity
of the name APICOMPLEXA, and adduced the following arguments in
support of her viewpoint:
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