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The
Diversity of The World of Life
Animals
(Metazoa)
Roundworms
(Nematoda)
Representatives
Roundworms,
or Nematodes,
including elephantiasis parasite, Guinea worm, heartworms,
hookworms, Loa loa (African eye worm), pinworms, root-knot
nematodes, Trichinella (trichinosis parasite), etc.
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
12,000
ENVIRONMENTS
Roundworms live in saltwater or freshwater, on or in moist soil, or on or in
plants or animals.
Nematodes are perhaps the second-most abundant animals on
Earth (second only to the arthropods in terms of the number of
individuals) -- roundworms outnumber us human beings perhaps a billion-to-one
and live on and in almost every living and nonliving thing on the planet (including you and me)! Fortunately,
most are harmless or actually helpful to us (as by feeding on bacteria or pest
nematodes).
DESCRIPTION
"Hydrostatic" support (water-pressure) -- 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 elongate, cylindrical body, tapering towards both
ends. The cuticle is often sculptured, ringed, or spined; and although
it grows in the adult (unlike the exoskeleton of arthropods), it is periodically
molted in the young (like the exoskeleton of arthropods). There may be an
adhesive gland at the tail-end. Nematodes
are typically microscopic to minute, but certain species parasitic within
vertebrates may grow up to several meters long!
FEEDING HABITS
Roundworms are scavenging, carnivorous, or parasitic (on or in plants or
animals).
MOTION
Roundworms have muscles running longitudinally (lengthwise)
within the
body wall. Most roundworms move with a characteristic side-to-side,
whip-like motion.
DIGESTION
The gut is "complete" -- it has both a mouth and
an anus. The mouth is sunken
within a "buccal cavity" (surrounded by such structures as
"lips" and bristles) and is often equipped with
"teeth" or a piercing/sucking "stylet".
Behind the mouth there is a muscular, food-pumping
"pharynx" (sometimes called an "esophagus"), with a
one-way valve at its rear end, to prevent regurgitation. Beyond
the pharynx, the gut consists of an "intestine" (in which food
materials are digested and from which food materials are absorbed), ending
in a "rectum" (which forms the feces).
RESPIRATION
Gases diffuse by osmosis (through
selective cell membranes) across the body wall -- there is no
respiratory system within these typically thin creatures, as there is in
higher animals.
CIRCULATION
The pseudocoel (the unlined body cavity outside of
the gut) carries materials circulating throughout the typically small body
-- there is no developed circulatory system, as in higher animals.
EXCRETION
"Protonephridia" (as in flatworms) or other
primitive excretory systems composed of small internal tubes, "driver" cells, and
external openings help maintain the body's salt- and water- balance -- there are no true kidneys.
COORDINATION
The growth, development, and activities of roundworms are
under genetic, hormonal, and/or nervous control.
A
"nerve ring" and "nerve cords" process nervous-system
inputs and outputs. Although a roundworm has a head, it is poorly "cephalized"
-- there are not a great many sensory organs in the head (In particular,
"endoparasites", which live within the body of their hosts,
typically have little need for elaborate navigation systems).
REPRODUCTION
Most roundworms reproduce sexually.
The reproductive organs typically take up much of the space within
the "pseudocoel" (the unlined body cavity outside of the gut) --
the reproductive organs of the female are typically paired, one set in the
front half of the body, the other in the rear.
The sexes of roundworms are almost always "separate":
Each individual has either male or female "gonads" -- either "testes"
(producing sperms) or
"ovaries" (producing eggs) -- but not both.
"Copulation" (mating with physical contact) takes place
in many species of roundworms. Typically,
the male uses his curled tail to grab onto the middle of the body of the
female. Then, his pair of
short, curved "spicules" reach out from his "cloaca"
(his rectum, functioning as the outlet to both the digestive and
reproductive systems) and pry open
her "vagina" (the female "gonopore", sexual opening).
His sperm (without propeller-like "flagella", as in most
animal species) -- which have been stored in his "seminal
vesicle" -- are then ejaculated by his "vas deferens" out through his
cloaca and into her vagina. The sperms are stored within her "spermatheca". The
eggs are later fertilized within the body of the female as they pass from the
ovaries through the spermatheca and via
an "oviduct" to her "uterus". The fertilized
eggs are stored temporarily within the uterus, before being released
through the gonopore. The eggs have a thick shell, which is often sculptured (and, thus,
useful in identifying species, such as parasites).
Parasitic species typically have one or more
"intermediate" hosts for their larvae, which are different from the
"primary" hosts for their adults, in their often complex life
cycles.
Animals
(Metazoa)
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