Platyhelminth Characteristics:
There is no single, unifying characteristic that defines the Platyhelminthes. For that reason, some suspect that the phylum Platyhelminthes is actually a
paraphyletic taxon. For the time being, though, we’ll consider them to be a true clade.
As mentioned, platyhelminths are acoelomate worms with dorsoventrally-flattened bodies. So far, that doesn’t distinguish them from the acoelomorpha. The internal anatomy of platyhelminths tends to be rather more complex than is true of the aceolomorpha, however, and unlike the acoelomorpha, the development of platyhelminths clearly identifies them as protostomes.
Because they lack any sort of waterproof outer covering that would prevent fatal dehydration, platyhelminths must live in water, or at least in wet environments. But whereas all known acoelomorphs live in water, there are a number of terrestrial platyhelminths.
As do acoelomorphs, a platyhelminth has an
incomplete digestive system, so the single opening into it functions as both the mouth and the anus. Some parasitic flatworm species lack digestive systems entirely.
Also like acoelomorphs, platyhelminths have true muscles, of all three types. As in the acoelomorphs, this gives them far more control over their movements than cnidarians or ctenophorans can manage, but the lack of any sort of hydrostatic skeleton means that few platyhelminths can burrow, nor are very many species capable of generating enough muscle force to swim.
The sense organs of platyhelminths are generally similar to those of acoelomorphs, and free-living species typically have gravity-sensing statocysts and light-sensitive ocelli. On the other hand, platyhelminths have rather more complex nervous systems than do acoelomorphs; indeed, the platyhelminths are frequently said to be the simplest animals with true nervous systems.
In a typical platyhelminth, neurons are organized into a pair of
anterior ganglia that function as a very simple “brain.” (A ganglion is a collection of neural cells.) Extending down the length of the body from each ganglion is a single
longitudinal nerve cord. The two nerve cords are connected by
transverse nerve cords that bridge them like the rungs on a ladder, so a platyhelminth is said to have a
ladder nervous system.
This is a much more centralized and complex nervous system than that possessed by any animal we have yet discussed. Such a nervous system allows for relatively quick and well-coordinated movements and responses, and also allows some degree of
learning. The flatworms commonly called “planarians” are frequently used in high school- or college-level biology courses to demonstrate learning, because they can be taught to navigate very simple mazes.
Most species of flatworms are monoecious and reproduce sexually. Nonetheless, many species can reproduce asexually, and some are famous for it. If you
did play with planarians in high school or college biology courses, you might have cut one in two. What do you get if you cut a planarian in two? You get two smaller planarians. If you slice one down the midline, the right half grows a new left half and the left half grows a new right half. If you cut one in two crosswise, the anterior end grows a new posterior end and the posterior end grows a new anterior end (including eyespots and anterior ganglia).
Those species of flatworms that are parasitic often have very complex life cycles. We’ll discuss some representative examples shortly.
The internal anatomy of a typical platyhelminth. Though it’s superficially
similar to an acoelomorph, its internal anatomy is much more complex.
Platyhelminth Characteristics
What are the symptoms of a liver fluke infection? I like sushi and has eaten plenty in my life. They were cooked though, or so I thought.
Linear Mode
