There are 5 main characteristics which put all the species within the Chordata phylum, they all have evolved to have a notochord within their bodies, they have this instead of a shell, this notochord when combined with the Dorsal Hollow Nerve Cord provides an internal support which the fluid tube of nerve fluid will then pass from top to bottom of the animal, acting as a spinal cord.
These are ancestors of humans as the boned spine of us today can be linked back to the Pikaia, the very first back boned animal. There is also a pair of openings through the pharynx present which shows that they are filter feeders, they all have blocks of muscle which extend into a post anus tail, the muscle surrounds the notochord along the whole length of the animal.
The basic traits of this phylum are that all the species are bilaterally symmetrical; they are segmented and have developed a gut tube, their coelom. The Devonian Period was a time in history where fish developed in leaps and bounds! They were first thought to have evolved in the Cambrian explosion, Levine (2003). The Pikaia fossil was found in the Burgess Shales, the fossil shows us many ways of how this organism lived.
We can tell that it swam along the sea bed using its tail as a fin, because of its mouth area, no scientist has distinguished what it exactly ate but it looks like it just fed off particles in the water and near the sea floor. Being one of the first fossils having “soft parts” it is a major discovery, Paleobiology.si.edu (2011).
Urochordata (Tunicates)
Sea Squirts are marine animals which both are solitary and colonial, depending on the species. They are sessile animals and have a unique feature; where they settle on a rock head first, their tail disappearing overtime and once stuck they then move their mouth moves back upward to catch passing particles to consume.
The hermaphrodite species release sperm and eggs. The eggs are about a third of one millimetre and kept in the body until they start to evolve within a few weeks. A siphon takes in the passing particles to be absorbed; this is known as suspension feeders.
Cephalochordata (Amphioxus)
Lancelet worms are filter feeders with no brain at all and are a marine subphylum, they swim and can bury themselves in the sand. All of the characteristics of a chordate are present, including the post anal tail. They do however have poor eyesight and lack of brain, but as they have sensors they are not really needed. So instead of wasting energy and time developing them they have evolved to have superior “feelers” and this allows them to hunt efficiently, Barnes (1994).
There are only about 30 species of Lancelet worm present they can grow up to about 7cm in length. Their cirri (tentacle feelers) are used as sensors for particles in the water, the pharynx has lots of gill slits which allows water to pass. Once absorbed the water passes out through the atriopore, Sherwood (1977).
Craniata (Vertebrates)
Ggnathastomes have two main types within; these are the Hagfish that secrete slime as a protection from predators and the Lamprey which have a cartilage dorsal cord and have seven gills present on each side. They are a parasitic species. The main difference is their habitat, Lampreys can live in both freshwater and oceanic but Hagfish can only live in Oceanic waters.
Their bodies have been developed to have a lower salt concentration in their bodies compared to in the waters which is why if put in a fresh water environment they wouldn’t survive. You know these are carnivorous because of their sharp toothed tongues, which pierce through their prey. The Hagfish can tie itself in a knot as a way to release itself from prey if caught and also to wipe the slime off of its body to make a quick escape. They were first thought to be hermaphrodites but research shows that this is not the case, they are born with both sex cells but only one takes dominance, Biglow (1948).
So there you go, I hope you enjoyed learning a bit about our wonderful world and those that inhabit it!
Holly Brega 