Echinoderms are starfish, brittle stars, sea cucumbers and sea urchins. Chordates are fish and birds, reptiles, amphibians and mammals. What is there to compare? What do they have in common? Surprisingly, echinoderms may in fact be the chordates’ closest invertebrate relatives.
Although superficially very different, when one looks at their embryonic development, they show some important similarities which indicates that they are actually closely related. The major differences between chordates and echinoderms are the lack of internal skeletons in echinoderms and their radial body plan.
Chordates are bilaterally symmetrical: the left half of the body resembles the right half. Starfish and other adult echinoderms are radially symmetrical, so they look quite different. However both echinoderm and chordate embryos start out bilaterally symmetrical and they share other important characteristics at the embryonic level.
The animal kingdom can be divided into two major groups: the protostomes and the deuterostomes. Most invertebrate phyla are protostomes, including the annelids, molluscs and arthropods. During protostome development, the first major opening in the developing digestive system of the embryo, the blastopore, becomes the mouth. In the two deuterostome phyla, echinoderms and chordates, the blastopore becomes the anus. This is a pretty fundamental difference and this separation in the two groups goes back some five hundred million years. This is when the two groups are thought to have separated.
There are other differences too. Protostomes develop their embryo by a process of spiral cleavage. Deuterostome embryos develop by radial cleavage. The first two divisions are identical in both groups: one cell divides into two and then two into four. After this, deuterostome cells divide in a radial pattern around a central axis, while protostomes divide in a spiral pattern. (Checkout http://www.utm.edu/departments/cens/biology/rirwin/radspircleave.htm for pictures of this process.)
Another fundamental difference between protostomes and deuterostomes is in the nature of the fate of cells. In echinoderms and chordates, the fate of cells is indeterminate, that is, if the first four cells are separated, each will grow into a separate organism. This is how identical twins can eventuate. In protostomes, the cells are determinate, meaning that their fate is determined before they are formed, so if the first two to four cells are separated, they cannot form whole organisms.
A third fundamental difference lies in the ways the two groups form the coelom or primary body cavity. Most protostomes are what are called schizocoelomates. This word means split body cavity. As the embryo develops the three body layers, ectoderm, endoderm and mesoderm, the body cavity forms when the mesoderm splits in two.
In deuterostomes, the endoderm or inner layer forms two pouches which become the coelom. The coelom in these organisms is called an enterocoelom, meaning gut-cavity, because the coelom forms from these gut pouches. So although adult echinoderms and chordates look and act very differently, at a fundamental level they are related and share these important embryonic characteristics: their blastopore becomes the anus, their cells are indeterminate, cleavage is spiral and they develop their primary body cavity from gut pouches. So next time you look at a starfish, consider the fact that it may be a close relative, regardless of how different it looks!
For more information: http://kentsimmons.uwinnipeg.ca/16cm05/1116/16anim5.htm
