As any worm enthusiast will quickly realize, there are many so-called “flat worms” in the animal
kingdom, but the animals of phylum Platyhelminthes were the first to earn that distinction. The name
Platyhelminthes comes from the Greek words platy, meaning flat, and helminth, meaning worm. Thus,
they are commonly referred to as flatworms. Though most of them are free-living, meaning they survive
in nature without a host, others are parasitic. All members of phylum Platyhelminthes
are triploblastic, meaning they have three unique cell layers. They are acoelomates,
meaning they lack any true body cavities. And they are bilaterally symmetrical, meaning their bodies
can be divided along a plane that splits the animal into a “left” and “right” side that are
approximate mirror images of each other. They all have incomplete guts, meaning they lack an anus,
and they demonstrate cephalization, meaning they have some type of head
with a centralized nervous system. This nervous system is comprised of an anterior
cerebral ganglion, somewhat like a “basal animal brain”, and longitudinal nerve cords connected by
transverse nerves. The head region of many flatworms also contains light-sensitive ocelli,
also known as light-sensitive eyespots, and a high concentration of chemoreceptors. Parasitic
flatworms also have abundant sensory organs around their oral suckers and holdfast organs.
Though flatworms have a nervous system and a complex reproductive system, they lack skeletal,
respiratory, and circulatory systems. This might seem bizarre at first, since they are animals
with three cell layers that lack blood. However, large flatworms have extremely flattened bodies,
which means that cells can more easily exchange molecules between each other and the surrounding
environment. The larger species also have a branched digestive system which helps with
circulation of material throughout the body. Essentially, all of their cells are only a short
distance from either the external environment, digestive system, or excretory system, allowing
nutrients to exchange through diffusion. This eliminates the need for a complicated circulatory
system, but restricts them to environments where dehydration is unlikely, such as seawater or
freshwater, moist terrestrial environments such as leaf litter or between grains of soil, and as
parasites within other animals. The majority of free-living flatworm species are small,
around one millimeter in length, and cylindrical. Most free-living flatworms have a ciliated
epidermis that sits on top of their body wall musculature, which has outer circular and inner
longitudinal muscles. Rod-shaped rhabdites that form a protective mucus sheath around the body
are produced by glands within the meshwork of their parenchyma cells, which are made up of
cells that developed from the mesoderm and fill the spaces between muscles and visceral organs. In
addition to these fixed cells, the parenchyma, or mesenchyme, also contains pluripotent stem cells,
which can transform into any other type of cell, and are used in regenerating tissues
after injury or asexual reproduction. Meanwhile, the surface of the epidermis
of many free-living flatworms contains single-cell mucous glands
along with dual-gland adhesive organs that are made of three cell types. These include adhesive,
or viscid glands, which allow the animals to chemically attach themselves to the substrate,
releaser gland cells that dissolve their adhesive and allow them to quickly detach their bodies,
and anchor cells that hold the apparatus in place. Most platyhelminthes have a digestive system that
begins at the mouth or pharynx, and terminates at the gastrovascular cavity or intestine, though
some, like tapeworms, digest food directly across their body walls and lack any form of a digestive
system. In many species, like fresh-water dwelling triclads, often called planarians, the mouth and
pharynx are not located in the anterior region, like in most bilaterally symmetrical animals.
They are instead located on the ventral side of the posterior region. In addition, the pharynx can
often extend out through the mouth, allowing the flatworm to feed and still move its head freely.
Platyhelminths have no anus and regurgitate undigested material through the mouth. Digestion
normally begins extracellularly, that is, outside of the organism’s cells, due to the secretion of
proteolytic enzymes that catalyze the breakdown of protein by cleaving peptide bonds. As the food is
slowly broken down, the flatworm will generally suck small pieces into the intestine, often
through a pharynx, where phagocytic cells will complete the digestion process intracellularly.
In addition to a digestive and nervous system, parasitic and freshwater flatworms also possess
excretory and osmoregulatory systems that are dually controlled by their protonephridia. The
invertebrate nephridium occurs in pairs and functions similarly to the vertebrate kidney.
That is, they remove metabolic waste from an animal’s body.
The protonephridium of the flatworms is a network of dead-end tubules that lack internal openings.
The dead-ends are called flame cells, which are cup-shaped and filled with beating flagella that
resemble a flickering flame. The beating of the flagella moves fluid through the animal,
excretes waste, and allows for osmoregulation. Almost all flatworms are monoecious,
or hermaphroditic, meaning they have both male and female reproductive organs.
And many are capable of both sexual and asexual reproduction. We will cover all of that in a
later tutorial, since it is quite different for parasitic and nonparasitic flatworms.
Traditionally, phylum Platyhelminthes was divided into the classes Turbellaria, Trematoda,
Monogenea, and Cestoda. However, this classification is now obsolete since the parasitic
trematodes, monogeneans, and cestodes are known to form a monophyletic group related to one of the
groups previously classified within Turbellaria. This means that the parasitic groups evolved from
a free-living ancestor. The four-class division of the Platyhelminthes is still often used in
many textbooks due to its simplicity, but it is now widely recognized as artificial.
Modern cladistics of Platyhelminthes reveal that the phylum is split into two sister clades:
Catenulida and Rhabditophora. Members of Catenulida have protonephridium and testis
unpaired, and nonmobile sperm. Members of Rhabditophora contain all parasitic flatworms
and the majority of the species once considered to be part of Turbellaria.
These flatworms are characterized by the ancestral presence of rod-like rhabdites that are absent in
Catenulida, and their dual-gland adhesive organs. Rhabditophora contains at least ten orders
of free-living flatworms and several parasitic orders. We will go over all of this diversity
over the next few tutorials, starting with the free-living flatworms before moving on to the
parasitic ones. So with an introduction to Platyhelminthes complete, let’s move
forward and cover the incredible diversity and regenerative ability of the free-living flatworms.