In the previous tutorial, we began discussing Nephrozoa, an extremely large clade containing
most of the animals you’ve ever heard of. This is divided into Protostomia and Deuterostomia.
Again, blastopores which develop into a mouth result in protostomes, whereas blastopores which
develop into an anus result in deuterostomes. For the next sizable portion of this series,
we are going to be focusing on all of these phyla within Protostomia, which fall into two
very large clades, Spiralia and Ecdysozoa, and we will begin with Spiralia. Unlike Ecdysozoa,
Spiralia is a difficult clade to define based upon morphology alone. Perhaps the two most
widely recognized Spiralian phyla are Mollusca, which is the second largest animal phylum,
and Annelida, which includes the earthworms and leeches. Other less familiar phyla are
the flatworms of Platyhelminthes, the ribbon worms of Nemertea, and the wheel animals of
Rotifera. However, these are just a few of the numerous Spiralian phyla, and as was promised
earlier in the series, we will be covering all of them, from the well-known mollusks that are found
all over the world, to the sac-like symbions that make their living on the mouthparts of lobsters.
The term “spiralia” refers to spiral cleavage, which is a pattern of development that occurs in a
developing embryo. Spiral cleavage begins after a zygote, or fertilized egg cell, begins to divide.
The first two divisions of the zygote result in four blastomeres, or macromeres, that each
represent a different quadrant of the embryo. Note that the spiral pattern is already forming, since
the cells do not align at right angles with one another. With each subsequent cleavage cycle the
resulting blastomeres give rise to an additional four cells, or micromeres, that are rotated
relative to their parent, resulting in alternating symmetry and the characteristic spiral pattern.
It’s worth noting that this isn’t the only way that spiral cleavage occurs.
We have just described equal cleavage, since the first two cellular divisions
produced blastomeres that were indistinguishable from one another. However, in unequal cleavage,
the first two cell divisions produce four cells in which one cell is bigger than the other three,
and thusly designated as the “D” blastomere.
Our current understanding of spiralia indicates three major clades; Gnathifera, which includes
small animals with chitinous jaws, such as the rotifers, jaw worms, and arrow worms, Rouphozoa,
which includes the flatworms and hairybellies, and Lophotrochozoa, which includes the segmented
worms, mollusks, and several others. This classification is somewhat complicated
by the fact that the term “lophotrochozoa” is sometimes used in place of “spiralia.”
This would make Lophotrochozoa the sister group to Ecdysozoa instead of Spiralia. However,
the clade Lophotrochozoa was originally defined as the “last common ancestor of
the three traditional lophophorate taxa, those being brachiopods, bryozoans, and phoronid worms;
the mollusks and the annelids; and all of the descendants of that common ancestor.”
Its name refers to lophophore or trochophore larvae, a characteristic that is present in many,
but not all members of spiralia. It unites the animals within the clade Lophotrochozoa but leaves
out all members of Gnathifera and Rouphozoa. Regardless of classification, since
spiral cleavage does not occur outside of Spiralia, it is considered to be a synapomorphy,
or derived characteristic unique to the clade, even though it has been lost in some members.
However, this loss of spiral cleavage is somewhat like the loss of limbs in snakes
and whales. Snakes and whales are considered to be tetrapods, or four-limbed animals,
even though they have lost some or all of their limbs. This is because they share common ancestry
with other tetrapod animals. They lost their limbs after they diverged from a common ancestor.
More recent analyses of large genomic data sets indicate that spiral cleavage was once
shared by all members of Gnathifera, and Rouphozoa, and Lophotrochozoa.
So as we can see, Spiralia includes a huge range of animals, so we will need many tutorials to get
through this clade, one phylum at at time. We will begin with phylum Platyhelminthes,
so let’s move forward and get an introduction to these parasitic and free-living flatworms.