The
Diversity of The World of Life
Animals
(Metazoa)
Flatworms
(Platyhelminthes)
Representatives
Planarians
etc. (Turbellaria)
Flukes
(Trematoda)
Tapeworms
(Cestoda)
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
13,000
ENVIRONMENTS
In salt- or fresh-water, in moist areas on land (as in
greenhouses), or in or on other animals.
DESCRIPTION
Flatworms have some true "organs" (specialized,
coordinated
assemblages of "tissues", composed of specialized cells) but no
"coelom" (membrane-lined body cavity outside of the gut) -- the
body is filled with "mesenchyme" (composed of loosely arranged
cells).
"Hydrostatic" support (water-pressure) gives
shape to the "bilaterally symmetrical" body (with mirror-image left and right
halves), typically "dorsal-ventrally"
flattened (that is, flattened from top-to-bottom).
The body of a turbellarian (such as the triangular-headed
planarians commonly found in greenhouses) is covered with tiny hairlike
"cilia". Although
some unusual terrestrial species are up to two feet long, most turbellarians are 10 mm or less in
length, often microscopic.
The body of flukes and tapeworms is covered with a tough,
yet absorptive "tegument".
Flukes are typically less than an inch long, but the segmented body
of a tapeworm may be over 10 meters long!
FEEDING HABITS
Turbellarians are "scavenging" (feeding on waste
matter), "herbivorous" (feeding on plants), or
"carnivorous" (feeding on animals); and flukes and tapeworms
are "parasitic", often causing severe internal injuries to their
"hosts" (such victimized creatures as us human beings).
MOTION
Flatworms can actively move about by means of their
well-developed muscle tissue, which develops in the "embryo" (early
young) from neither the "ectoderm" (producing the skin)
nor the "endoderm" (producing the gut) but rather from the
"mesoderm" (an embryonic layer that, in higher animals, also
produces the other major internal tissues and organs).
Turbellarians secrete a film of mucous to
smooth their path, which they glide over with their hair-like cilia.
DIGESTION
Although absent in some species (See below), the gut of
a flatworm is saclike and often branching, into both sides of the body.
There is a mouth (opening in the middle of the underside of the
body in such turbellarians as planarians) typically connected to a
muscular "pharynx" (which may be pushed-out through the mouth in
some species); however, there is no anus (Digestive wastes are discharged
through the mouth).
The gut has been lost in the evolution of such flatworms
as tapeworms. These parasites simply absorb nutrients, through
their tough, yet absorptive outer tegument, from
within the body of their hosts. Tapeworms are typically found as parasites within the
intestines (lower gut) of their vertebrate hosts, in which they are anchored by the
suckers and/or hooks of their "scolex" (a tiny bulbous structure
at the "head" end of their body).
Parasitic flukes typically attach themselves to the outside or
inside of their hosts by means of suckers.
RESPIRATION
Gases simply diffuse by "osmosis" (through
selective cell membranes) through the typically broad and flat body wall.
CIRCULATION
There is no developed circulatory system -- dissolved
materials simply diffuse by osmosis between cells, as through the mesenchyme,
within the small body.
EXCRETION
Wastes rich in nitrogen (as found in our urine)
simply diffuse by osmosis across
the body wall of most flatworms.
In freshwater turbellarians and similar species, excess
water continually absorbed from the environment is
expelled by one to several pairs of "protonephridia" (primitive
kidney-like structures): Water
is collected in a system of tiny, typically branching tubes within the
body; driven by the "flickering" hair-like "cilia" or
propeller-like "flagella" within "flame cells"; and
expelled out a pore on the surface of the body.
This primitive "excretory" system also helps control the
delicate balance of dissolved salts within the body.
COORDINATION
The activities, growth, and development of flatworms are
under genetic and hormonal control.
Flatworms possess nerve "cords" (functioning like electrical
cables) and a
primitive "brain" (functioning like an on-board computer), which processes
inputs from such sensors as the eyespots on the typically triangular head
of a planarian -- these primitive eyes can sense lightness or darkness
but cannot see distinct images of shapes.
"Cephalization" (the development of a head) has
evolved amongst most animals, as an aid to mobility:
Many sensors on the front end of the body help to safely and
efficiently direct the creature moving forwards.
Most "endoparasites" (parasites that live entirely within
the body of their hosts) have little need for such environmental sensors
(or legs, etc.); and as such, they typically have poorly developed heads
(The so-called "head" of a tapeworm is devoted mostly to just
hanging onto the gut of its host).
REPRODUCTION
Many flatworms can reproduce asexually. Such
turbellarians as freshwater or terrestrial planarians simply divide in two
crosswise -- the head end re-grows a new tail, and the tail end re-grows a
new head!
Many flatworms are "hermaphroditic" -- each
individual has both male and female "gonads", structures
producing the "gametes" (sex cells, each with half the normal
number of chromosomes).
Although they could theoretically mate with themselves (which, as
the highest form of "inbreeding", would tend to decrease the
spread of helpful genetic traits throughout their population and increase
the influence of harmful traits), flatworms typically seek out mates, for
"mutual fertilization": During
"copulation", the penis (male sex organ) of each
partner puts sperm (male gametes) into the vagina (female sexual opening) or a "common gonopore"
(an opening
serving both the male and the female reproductive systems) of the other
partner; and the sperm uses its propeller-like "flagellum" to
swim in to the egg (female gamete).
For turbellarians, the fertilized eggs (each with
both sets of chromosomes) are often deposited within a mass of "jelly" or a protective
"cocoon".
The body of a tapeworm is composed of a chain of "proglottid"
segments, each of which contains a complete "hermaphroditic"
reproductive system (that is, each segment is both male and female):
Sperms from any given proglottid fertilize eggs from any other
proglottid (on the same worm or another); and typically, the older, larger proglottids, at the
"tail" end of the tapeworm, eventually break off, allowing the
fertilized eggs within to be passed out of the digestive tract of their
host and into the environment. The
"embryos" (early young) and the often free-swimming, ciliated
"larvae" (later young) of tapeworms develop in one or more
"intermediate hosts" (For example, there is a species of tapeworm that can infect human beings who eat undercooked beef from
cattle -- the intermediate hosts -- that have been infected with the hard
"cysts" of tapeworm larvae).
The life cycles of parasitic flukes and tapeworms are
usually complicated, requiring different "hosts" for different
life stages and involving both asexual and sexual reproduction.
Throughout the Animal Kingdom, parasites typically produce enormous
numbers of gametes, which increases the odds that at least some of their offspring
will survive, particularly out in the harsh environment.
Animals
(Metazoa)
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