At this point in the series, it’s time to start digging into Nephrozoa, which contains most of the
animals we are likely familiar with. But before doing so, we have one more rather interesting
phlyum to discuss, and that is Xenacoelomorpha. The animals in phylum Xenacoelomorpha are widely
considered to be the sister group to all other bilaterally symmetrical animals, that is, every
animal on the cladogram we haven’t covered yet. So far, we’ve studied the phyla Porifera, Placozoa,
Cnidaria, and Ctenophora. All of these phyla fall outside of the traditional classification
of triploblastic Bilateria. This term includes animals that have a left and right side that are
mirror images of each other, and three unique germ cell layers, the ectoderm,
mesoderm, and endoderm. All Nephrozoans are further split up into the three superphyla
we discussed in the beginning of this series, namely, Deuterostomia, Ecdysozoa, and Spiralia.
You’ll notice, however, that there is one phylum that is part of the triploblastic Bilateria,
but outside of Nephrozoa. That is phylum Xenacoelomorpha. These unassuming,
“worm-like” creatures actually play a key role in understanding the rest of kingdom animalia since
they are considered to be the most basal group of bilaterally symmetrical triploblastic animals.
This is due in part to the fact that they, like the cnidarians,
have only one opening, but like almost all other animals, they are bilateral triploblasts.
Phylum Xenacoelomorpha is further divided into the sister clades: Xenoturbellida and Acoelomorpha.
Xenoturbellida contains a single genus Xenoturbella and six species, including
the recently discovered Xenoturbella churro, named as such for its resemblance to the famous
fried dough pastry, and Xenoturbella monstrosa which can grow up to 20 centimeters in length.
All members of this clade are dorsoventrally flattened, and possess a ciliated ring furrow,
a thick epidermis, longitudinal, circular, and radial muscles,
a ventral mouth that opens into the gut, and an anterior balance sensory receptor
known as a statocyst that is used for orientation. They all lack a circulatory,
respiratory, and nervous system and have no defined organs except for the statocyst.
The acoelomorphs are tiny flat worms less then 5 millimeters in length that are often found
living between grains of sediment, floating as planktonic organisms,
or crawling over the surfaces of corals.
Like the xenoturbellidas the acoelomorphs have a ciliated epidermis and a statocyst. However, some
acoelomorphs also have pigment-spot ocelli, or simple eyes, that are capable of detecting light.
This group contains about 350 species, most of which are in class Acoela which contains
the shilly-shally, or mint-sauce worm. These animals are native to the Atlantic coast of Europe
where they have been observed from Wales to the South of Portugal. Their bright green pigmentation
intrigued scientists in the 1870s, and they have been the subject of numerous studies ever since.
It turns out that the shilly-shally worms get their green pigmentation
from the interaction of their own bodies and the symbiotic microalgae, Tetraselmis convolutae,
that they host under their epidermis. Juvenile worms are born without photosynthetic symbionts
and must obtain them indirectly from the environment through their ventral orifice that
functions like a mouth. However, at no point in their development do they have a digestive system.
So, unlike the hermatypic corals that rely in part on photosynthetic symbionts,
the shilly-shally worms rely entirely on their symbionts to survive. Juveniles that are
incapable of collecting enough microalgae or are experimentally prevented from doing so, will die.
And with that we’ve now covered every animal phylum outside of the major clade Nephrozoa.
And since this clade contains almost all of the animals you’ve ever heard of,
you can probably imagine that we are just getting started.
Let’s move forward and continue exploring the animal kingdom.