blastopore becomes a mouth and who have spiral and
determinate cleavage.
protozoan Thesimplest forms of animal life, con-
sisting of aquatic single cells or colonies of single cells,
such as amoeba and paramecium. The “protists”
ingest food and live in both fresh and marine waters as
well as inside animals, and most are motile. Estimates
of the number of species of protozoa are between
12,000 to 19,000 species. Many protozoans are
known to cause human disease, such as Giardia lam-
blia,a flagellated organism that can infect via water or
be contracted via contaminated foods. It causes giar-
diasis, the most frequent cause of nonbacterial diar-
rhea in North America. Cryptosporidium parvumhas
a strong association between cases of cryptosporidiosis
and immunodeficient individuals (such as those with
AIDS). Cyclospora cayetanensisinfection results in a
disease with nonspecific symptoms: usually one day of
malaise, low fever, diarrhea, fatigue, vomiting, and
weight loss. Cryptosporidium parvumcauses intesti-
nal, tracheal, or pulmonary cryptosporidiosis. Acan-
thamoebaspp., Naegleria fowleri,and other amoebae
are responsible for primary amoebic meningoen-
cephalitis (PAM). Naegleria fowleriis associated with
granulomatious amoebic encephalitis (GAE), acan-
thamoebic keratitis, and acanthamoebic uveitis. Enta-
moeba histolytica causes amoebiasis (or amebiasis),
resulting in various gastrointestinal upsets, including
colitis and diarrhea. In more severe cases, the gastroin-
testinal tract hemorrhages, resulting in dysentery.
Other protozoans such as Naegleria fowleri cause
meningitis; Trypanosoma cruzicauses African sleeping
sickness and Chagas disease; and Plasmodium spp.
causes malaria.provirus Term given to a retrovirus DNA when it is
integrated into the infected host cell genome. It can
remain inactive for periods of time and can be passed
to each of the infected cell’s daughter cells.proximate causation Explains how organisms
respond to their immediate environment (through
behavior, physiology, and other methods) and the
mechanics of those responses.pseudocoelomate Any invertebrate whose body cav-
ity is not completely lined with mesoderm. The embry-
onic blastocoel persists as a body cavity and is not
lined with mesodermal peritoneum (the lining of the
coelom); therefore it is called a pseudocoel (“false cavi-
ty”). Examples include nematodes, rotifers, acantho-
cephalans, kinorhynchs, and nematomorphs.pseudopodium Adynamic protruding structure of
the plasma membrane used by amoeboid-type cells
used for locomotion and phagocytosis.pseudopodium 279Amoebas are protozoans, which are the simplest form of animal
life. Photo of Amoeba proteus. (Courtesy of Hideki Horikami)
The Vorticellais a protist (protozoan) and belongs to the phyllum
Ciliophora. A protist that turns as it moves.(Courtesy of Hideki
Horikami)