With Douglas Drenkow

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The Diversity of

The World of Life

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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)

Doug@DouglasDrenkow.com

(c) 2004 D.D.  All Rights Reserved.

Photo of Cells:  H.D.A. Lindquist, US EPA