Phylum
Platyhelminthes
Phylum Trend Synopsis: Commonly called flatworms, the phylum name is derived from
Greek and describes the body shape of these animals: dorsally ventrally
flattened but otherwise worm-like. The approximately 20,000 species are
classified into four classes: the free living flatworms in the Class
Turbellaria, the parasitic monogeans in the Class Monogea, the parasitic
flukes in the the Class Trematoda, and the parasitic tapeworms in the Class
Cestoidea. The body plan is that of an elongated organism that is dorsally
ventrally flattened with bilateral symmetry and with a hint of cephalization.
Distinct tissues and organ systems are found, but the organs are
not located in a body cavity. Cross sections of these animals show that, except
for the digestive cavity, no other internal cavity is found. Because the organs
are in direct contact with a surrounding loosely organized tissue, these
animals are said to be acoelomate. These animals are saoid to be
triplpoblastic because three layers of cells can be seen: an outer epidermis
(ectoderm), a middle layer that may be several cells thick (mesoderm)and a
distinct lining of the gut the endoderm. The digestive system is described as incomplete
because it is not a tube with a mouth at one end and anus at the other. There
is only one opening, so it is described asa sac-like system. Food enters
the mouth and is digested in a branched gastrovascular cavity. Any
non-digestable residue must be regurgitated back through the mouth. Muscle
layers are well developed and controlled by a distinct nervous system with two
longitudinal nerve cords that run the length of the body. Two nerve ganglia and
sensory receptors at the anterior end coordinate activity sending nerve signals
to the rest of the body by two ventral longitudinal nerve cords. There is
no separate skeletal system and body support is by hydrostatic pressure or by a
cuticle. There are no special organs for respiration and circulation. Diffusion
across the body surface or from the gastrovascular cavity to the body cells
fulfills these functional needs. Individuals may have both male and female sex
organs, producing sperm and eggs. Fertilization is internal. Microscopic
fertilized eggs are released. Development may be direct into minature adult
forms or indirect through a indepemdent larval stage. As parasites of animals,
the platyhelminthes have substantial impacts on ecosystems.
Modern analysis of DNA sequences indicates that these animals may
be related to the protostome branch of the animal phylogentic tree. They are
placed in a newly defined clade called the Lophotrochozoa along with rotifers,
annelids, and molluscs. This new grouping suggests that the lack of a
body cavity may not be a primitive condition but one that evolved secondarily
(later) from animals having a coelom.
Class Turbellaria
Class Turbellaria has approximately 3000 species in 12 orders. Most
are marine and benthic, but some also inhabit fresh water and moist temperate
and tropical terrestrial habitats. The order Temnocephalida is entirely
commensal or parasitic, but some members of other orders are commensal as well.
Classification within the Turbellaria is in a state of flux as it is unclear
whether the class is truly monophyletic. Currently orders are defined by the
type of pharynx, organization of the reproductive system, and branching pattern
of the gut. Orders of the class Turbellaria recognized by Ruppert and Barnes
(1994) are: Nemertodermatida, Acoela, Catenulida, Haplopharyngida,
Macrostomida, Polycladida, Lecithoepitheliata, Prolecithophora, Proseriata,
Temnocephalida, Rhabdocoela, and Tricaldida. Brusca and Brusca (2003) include
Temnocephalida as a suborder of Rhabdocoela, and recognize an additional class,
Proplicastomata, which includes known specimens from Greenland.
Like other members of the phylum Platyhelminthes
(flatworms), those of the class Turbellaria are dorsoventrally compressed, with
high surface area to volume ratios. Marine species can be quite colorful, but
the interstitial and terrestrial turbellarians tend to be drab. Turbellarians
generally locomote by coordinated waves of cilia on a secreted mucus trail,
though some species can swim by rhythmic muscle contractions. Their ciliated epidermis,
the presence of sub-epidermal
rhabdites, and their free-living condition distinguish turbellarians
from members of the other classes of Platyhelminthes.
The visual impression of their epidermal ciliary activity gives this group its
name, as 'Turbellaria' is derived from the Latin term for
"whirlpool."
Turbellarians lack fluid transport systems, and are acoelomate. The
gut in turbellarians has only a mouth opening. There is usually a pharynx for
introducing food into the gut, and undigested food particles are ejected
through the mouth. The final stages of digestion are intracellular. Most
members of class Turbellaria are predatory on invertebrates smaller than
themselves; the rest are herbivores, ectoparasites, or scavengers.
Turbellarians use protonephridia
scattered throughout their epidermis for the excretion of metabolic waste.
Turbellarians sense their environment with statocysts,
chemoreceptors, and photoreceptors. They do not have image-forming eyes, but many
species have pigment cells and photoreceptors concentrated into eyespots. The
complexity of the turbellarian peripheral nervous system varies from a simple
nerve net based on pairs of longitudinal nerve cords to an interlacing web of
nerves capable of fine muscular control. Turbellarians are simultaneous
hermaphrodites and lay eggs bundled into cocoons. The young undergo direct
development and hatch as juveniles. Spiral cleavage is prevalent. Some
turbellarians can reproduce asexually by fission. Regeneration of somatic parts
is well documented in Dugesia.
Class trematodes
All trematodes are parasitic, and most adult trematodes parasitize
vertebrates. Around 9000 species have been described. Their body is covered
with a tegument, a peculiar kind of epidermal arrangement in which the main
cell bodies are deep, separated from the cytoplasm that lies next to the
exterior by a layer of muscle (but connected to the exterior layer by cellular
processes. The exterior layer is syncytial; that is, it is continuous, not
broken by cell membranes. The tegument lacks cilia in adults. Unlike
monogeneans, trematodes have no opisthoaptor; instead, they are characterized
by one or two suckers. They are like turbellarians in having a relatively well
developed alimentary canal, and their muscular, excretory, and reproductive
systems are also relatively complete.
Most trematodes have complex life cycles, with larval stages
parasitizing one or more species that are different from host of adults. Larval
stages of some medically important species include miracidium, redia, cercaria,
and metacercaria. Most trematodes are endoparasites. They include several
parasites that have an enormous impact on human populations, such as human
liver flukes and the blood flukes that cause schistosomiasis.
Class cestodes
The cestodes, or tapeworms, differ in a number of ways from other
flatworms. Their bodies are long and flat, made up of many segments called
proglottids. Each proglottid is a reproductive unit, essentially a factory to
produce gametes. Adults lack cilia and their surface is a tegument (as in
monogeneans and trematodes), but in cestodes the tegument is covered with tiny
projections, microvilli, which increase its surface area and thereby its ability
to absorb nutrients from a host. Digestive tracts are absent completely. At the
tapeworm's anterior end is a specialized segment called a scolex, which is
usually covered with hooks or suckers and serves to anchor it to the host.
All of the 5000 or so known species of tapeworms are endoparasites.
Most require at least two hosts, with the host of the adult tapeworm a
vertebrate. Intermediate hosts are often invertebrates. A number of tapeworm
species inhabit humans.
Class monogenea
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Monogenetic flukes
spend their entire life cycle as parasites on a single host, often on the gills
and skin of fish; they include no human parasites. They hold on to the fish by
the use of hooks and attachment organs at the posterior end. Most of the
parasite's body space is devoted to the hermaphroditic reproductive system. The
egg on hatching releases a ciliated larva that enables the parasite to reach a
new host. Species of the genus Gyrodactylus can can be serious pests in
hatcheries, particularly since a single worm can give rise to more than one
hundred descendants in three weeks.
