In the previous tutorial we introduced some general characteristics of phylum Platyhelminthes.
Now let’s dig into the clades with this phylum. The free-living flatworms were traditionally
grouped as class Turbellaria, alongside two to three classes of parasitic flatworms.
However, more recent analyses have determined that the parasitic flatworms,
like the flukes and tapeworms, evolved from one small sub-group within,
not alongside, the free-living Platyhelminthes, formerly the Turbellarians,
making the old classification paraphyletic. In this tutorial we will focus on the
non-parasitic members of clade Rhabditophora, that is all flatworms outside of Neodermata
that secrete mucous using their rod-like rhabdites. These include a range of smaller taxa
with diverse body-shapes that range from the basal flatworms of order Macrostomida,
to the droplet-shaped, and sometimes colorful aquatic flatworms of order Prolecithophora,
to the contested clade of Rhabdocoela. This one includes the transparent, predatory Mesostoma and
the bizarre almost medusa-shaped Temnocephalida that are exclusively parasitic or commensal,
though they differ phylogenetically from the flukes and tapeworms of Neodermata.
So as we can see, there is an enormous amount of diversity when it comes to flatworms.
However, two of the most biodiverse clades outside of Rhabdocoela,
and those which have received the most attention from researchers, are Polycladida, and Tricladida.
Polycladida includes some of the largest and most strikingly colored free-living marine flatworms.
Members of this clade have been found from coastal shorelines all the way down to the
oceanic abyss. It is here that we can behold the nearly hallucinatory colors of the gold-speckled
flatworm, and the brilliant colors of the now famous Pseudobiceros that engage in traumatic
insemination, or “hypodermic impregnation”, a type of reproduction in some invertebrates where males
transfer sperm to females by literally piercing the female’s body and injecting sperm into the
wound. This is also known as “penis-fencing” where the hermaphroditic worms “fence” using their
extendable two-headed pointed stylets, and attempt to pierce their mate’s epidermis. In some species,
like the Persian carpet flatworm, even though both animals are hermaphroditic, and carry both
sperm and eggs, the first to penetrate its mate’s skin releases sperm into the wound and fertilizes
its eggs without having its own eggs fertilized. In this way, the “winner” becomes “male” for the
pairing and the “loser” becomes “female”. The “male” worm may leave to attempt to fertilize
other worms or be fertilized itself, while the “female” takes care of its fertilized eggs.
In other species, like the tiger flatworm, sperm is often bilaterally
transferred between both partners in unison. Along with their unique mating rituals, the marine
flatworms also have a range of defenses. Some species have been observed to acquire and store
Cnidarian stinging cells, or nematocysts from their prey. Others have been found to produce
defensive acidic secretions. And still others arm themselves with specular skeletons. However,
none of these adaptations rival the astounding regenerative power that has been extensively
studied not in the marine flatworms, but in some of the least assuming species of freshwater
free-living triclads, commonly called planarians. Planarians, or triclads, are one of many species
of free-living flatworms traditionally placed in Turbellaria, now considered to be
part of the order Tricladida. They include large carnivorous, neurotoxin-producing,
land-dwelling species like the hammerhead worms and the head-plated arrowhead worms, along with
the freshwater dwelling Dugesia, the cave-dwelling Dimarcusidae, and many others. However,
since members of Dugesia are considered to be the type species of their family and have undergone
more studies than any other planarian, they will be the focus for the remainder of this tutorial.
Nearly all planarians are carnivorous predators, or opportunistic detritivores. Though most do have
light sensitive ocelli, or eye spots they likely cannot see more than simple shadows and the change
of light. Most species have two auricles which resemble earlike projections at the base of the
head, that contain tactile cells, chemoreceptor cells, and statocysts, which allow them to smell
their environments, react to touch, detect the flow and intensity of the water current,
and maintain equilibrium. Planarians can reproduce both sexually and asexually. In
sexual reproduction, hermaphroditic planarians take part in bilateral transfer, similar to
the marine flatworms. Their eggs develop inside their body and are shed in capsules.
Weeks later, the eggs hatch and grow into adults. However, planarians and many other free-living
flatworms can also reproduce asexually through fission where they constrict behind their
pharynx and separate into two animals, called zooids, each of which regenerates the missing
parts. It is this ability of the flatworms that has captivated researchers for hundreds of years.
Though the first recorded descriptions of animal regeneration began with Aristotle’s observation of
lizard tail regrowth in 350 BCE, the power of planarian regeneration was unknown to science
until the late 1700s, when researchers began to find their vivisected bodies moved without their
heads. However, the earliest known reference to planarian regeneration came from the Chinese text
Yu-Yang Tsa-Tsu written around 860 CE by T’uan. Planarians, specifically species of Dugesia,
have been widely used as a model system in biological and biomedical research. A landmark
study on planarian regeneration came about in 1898, when Thomas Hunt Morgan found that a single
planarian could be cut into 279 pieces and each piece could grow into a new animal. This study
and many others launched a wave of research into planarian regeneration that continues to this day.
In 2011, it was discovered that over 20% of the cells in a planarian’s body are pluripotent stem
cells that have the ability to create all the various cell types planarians need for survival.
If just one 279th of a planarian’s body contains just a handful of pluripotent stem cells,
called neoblasts, or endoblasts, they have the capacity to create a new organism.
These neoblasts alone are responsible for the creation of new cells, no other cells within
the planarian body can make copies of themselves. In addition, a planarian does not need to be fully
severed to wield the power of its stem cells. If the head of a planarian is cut in half down its
center, and each side is retained on the organism, it is possible for the planarian to regenerate two
heads, or many more. When a planarian loses its head or its tail, pluripotent stem cells multiply
and differentiate to replace the missing part. The stem cells determine which part to replace
because they receive molecular signals from nearby differentiated cells. These signals regulate
differentiation into the specific cell types required to regenerate the correct body part.
In this way, even a tiny piece taken from the middle of a planarian
can regenerate both a new head and tail, that will be oriented in the same direction as its parent.
Though planarians are not the only animals capable of incredible feats of regeneration,
as we know from learning about sponges and immortal jellyfish earlier in the series,
the fact that many of their cell types, tissues, and simple organs are homologous to our own,
has made them an important part of a wide range of studies. And with that,
we can leave the free-living flatworms behind and begin our study of the parasites in this phlyum.