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The
Diversity of The World of Life
Animals
(Metazoa)
Gastrotrichs
(Gastrotricha)
Representatives
Gastrotrichs
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
175
ENVIRONMENTS
Gastrotrichs live in saltwater or freshwater.
DESCRIPTION
"Hydrostatic" (water-pressure) support -- within
the "pseudocoel" (the unlined body cavity outside of the gut) --
as
well as the tough, hard-to-stretch, yet flexible outer "cuticle" give shape to the
elongated, microscopic body.
The cuticle is typically composed of scales, which are sometimes spiny.
The
"ventral" surface (underside) is flattened and
"ciliated" (microscopically "hairy"); the poorly
defined head may have bristles or tufts of cilia; and the tail-end may
have a pair of adhesive tubes, rows of which may also be present on the sides
of the body.
FEEDING HABITS
Gastrotrichs are scavenging (feeding on wastes), herbivorous
(feeding on plants), or carnivorous (feeding on other animals).
MOTION
The cilia on the underside of the body allow the animal to
glide over the bottom. Muscles
also provide bodily motion.
DIGESTION
The gut is "complete" -- it has both a mouth and
an anus. Food is brought into
the mouth by the action of both the cilia and a muscular, pumping
"pharynx" (throat). The
intestine, which holds and digests foods, ends in a "rectum",
which
collects wastes.
RESPIRATION
Gases diffuse by osmosis across the 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 gastrotrichs are
under genetic, hormonal, and/or nervous control.
Although a gastrotrich 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
Most freshwater species of gastrotrichs reproduce
asexually.
The male reproductive system does not develop; and the resultant
female gastrotrichs reproduce "parthenogentically" (without
sexual fertilization of their eggs), which allows populations to build-up quickly
-- a distinct advantage for life in the uncertain conditions of inland waters.
In addition, eggs may remain dormant during periods of cold or dry
weather.
Gastrotrichs
are "hermaphroditic" -- each individual has (at least the
potential to develop) the "gonads" (producing the
"gametes", sperms or eggs) of both sexes.
Animals
(Metazoa)
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