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The
Diversity of The World of Life
Animals
(Metazoa)
Rotifers
(Rotifera)
Representatives
Rotifers
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
1,500
ENVIRONMENTS
Most rotifers live in freshwater, but some live in saltwater or in watery films
on mosses. Various species of rotifers are "pelagic"
(swimming); "planktonic" (drifting); or "benthic"
(bottom-dwelling), either crawling or tube-dwelling.
DESCRIPTION
"Hydrostatic" (water-pressure) support within
the "pseudocoel" (unlined body cavity outside of the gut) as
well as the tough, hard-to-stretch, yet flexible outer "cuticle"
give shape to the microscopic, generally cylindrical body. There is a
ciliated ("hairy") "crown" atop the head and a tapering, sometimes
telescoping "foot" (sometimes with a pair of adhesive
"toes") at the tail-end.
FEEDING HABITS
Various species of rotifers are either passive
filter-feeders or active predators.
MOTION
The crown cilia may be used in swimming; the muscles within
a rotifer, in leech-like
crawling.
DIGESTION
Unlike that in lower animals, the gut of a rotifer is
"complete" -- it has both a mouth and an "anus" (an
opening for the discharge of digestive wastes).
The beating cilia of the crown typically propel a current of food particles
from environmental water
into the mouth. There is a muscular "pharynx" (throat), bearing a
"mastax" (an organ with seven "teeth" that grab or
grind food). The gut consists
of a large "stomach", which primarily holds food, and a short
"intestine", which primarily digests food (chemically and
physically).
RESPIRATION
Gases diffuse by "osmosis" through the selective cell
membranes of the body wall -- there is no true respiratory
"system", as in higher animals.
CIRCULATION
The pseudocoel (the unlined body cavity outside of the gut)
carries materials circulating throughout the body -- there is no developed circulatory
"system", as in higher animals.
EXCRETION
"Protonephridia" (primitive, tubular excretory
structures, as in flatworms) or
related structures help maintain salt- and water-balance -- there are no
true "kidneys".
COORDINATION
The growth, development, and activities of rotifers are
under genetic, hormonal, and/or nervous control.
Although a rotifer has a head, it is poorly "cephalized"
-- there
are not a great many sensory organs in the head, as there are in most
higher animals.
REPRODUCTION
Many
freshwater species of rotifers can reproduce asexually; moreover, large populations may build-up quickly,
by means of "parthenogenesis" (reproduction without sexual fertilization of
the eggs) -- a distinct advantage for life in the uncertain conditions of
inland waters. Additionally,
freshwater species can produce thick-shelled, dormant eggs, to survive
periods of cold or
dry weather; and adult rotifers that live on mosses can survive in
a state of dormancy during periods (even years) of drought or low temperatures
-- down to hundreds of degrees below zero!
The
sexes of rotifers are "separate": Each
individual has either male or female "gonads" (organs producing male or female "gametes", sperms or eggs) but not
the gonads of both sexes.
The penis of the male injects sperm into any part of the body of
the female, who lays the fertilized, shelled eggs either on the bottom or on her body.
Animals
(Metazoa)
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