Representatives of the phylum Echinodermata are common inhabitants of coastal tide pools (you can see some of them in the saltwater aquarium at the HUB student union on the University Park campus). Sea lilies, sea stars, sea cucumbers, and most other echinoderms are sessile or slow-moving animals. Some representatives are instantly recognized by their five-fold symmetry (having rays and/or arms in fives or multiples of five). Even though the seemingly radial symmetry of these adult animals would lead one to question their placement in the bilateria, the bilateral symmetry of the larval stage of these animals indicates that they are members of the bilateria lineage. Furthermore, these animals are coelomates since they have a large, fluid-filled cavity lined with mesoderm and usually a complete gut. Many species of sea stars have the ability to turn their stomachs inside out by everting them through their mouths. This allows them to initiate the digestion of their prey before food enters their digestive tract.
A thin skin overlaying a hard, yet flexible, endoskeleton composed of calcium carbonate plates and spines characterizes the exterior of echinoderms (the name means spiny skin). These plates have a very complex arrangement. Electron microscopy of the surface of a sea urchin demonstrates this elegant arrangement, a sponge-like mesh which creates a plate that allows for special structures to protrude through the endoskeleton for locomotion, feeding, gas exchange, and protection. These protrusions, typically described as tube feet (sucker-like appendages), enable echinoderms to move and provide them with the ability to grip and manipulate objects or prey.
Echinoderms also have a unique water vascular system (a network of hydraulic canals extending in from the tube feet and around the gut of the organism). This expansive network uses hydraulic pressure to manipulate the extension of the tube feet as the animal breathes, feeds, or moves across the ocean floor.
Members of the phylum Echinodermata are diverse. In the adult state it is often unclear how these animals are related. However, a close inspection of their anatomy and embryology reveals their common evolutionary history. Shown here are members from four echinoderm classes (you will not be tested on these classes).
The relationships between humans and sea urchins may not be obvious, however, common embryonic features reveal their common ancestry. As you have learned, stages of embryonic development have an important place in the classification of animal phyla. Whether it is the fate of the blastopore, the number of germ layers (diploblastic versus triploblastic), or the formation and origin of the tissue lining the body cavity, embryonic origins provide information about the relationships among members of the Kingdom Animalia.
The close phylogenetic relationship of humans (and other chordates) and echinoderms is also supported by DNA sequence data. As more data are collected, these relationships will be refined, but to date the evidence of the relationship between the species of these groups is compelling. Some scientists have even discussed placing Phylum Echinodermata closer to Phylum Chordata, or even within Phylum Chordata, due to the discovery of some early echinoderms that might have possessed pharyngeal slits and a tail (diagnostic chordate features). Furthermore, undiscovered species may also provide information on the relatedness of chordates and echinoderms. From what you have learned, would a modern representative of these groups shed more light on this question, or could just as much be learned from the fossil of an adult animal? Why or why not?