Echinodermata — five-rayed and marine

This is the animal phylum that shows evolutionary relationships with the phylum Chordata. Starfish, brittle stars, sea urchins, sea cucumbers and sea lilies belong to it.

Figure to be added

Figure 13.24 — several species of Echinodermata: starfish, sea urchin and sea cucumber.

All live in sea water environments only.
Triploblastic, bearing a coelom. The body is divided into five radial arms.
They bear a covering with sharp spines.
They are star-shaped, cylindrical or flower-shaped.
A system of canals runs throughout the body. This is called the water vascular system.
They have tube feet for movement and respiration.
They bear no brain, no heart and no eyes.
The body shows pentaradial symmetry.
They show sexual dimorphism and reproduce sexually.

Figure to be added

Figure 13.25 — a cross section of an echinoderm's body: the water vascular system, the radial arms and a tube foot.

Radial symmetry, twice over

Cnidarians show radial symmetry too, but the two are not the same thing. An echinoderm's is pentaradial — five arms — and it is absent in the larva. Their larvae are bilateral, which is what shows the relationship with the chordates.