Sketch: Amphioxus and Brittle Star

Here is a sketch I made of an Amphioxus or lancelet, and of a brittle star (class: Ophiuroidea). It was once thought that Amphioxus was the closest living relative of vertebrates, but now it looks like the sea squirt or Tunicate (as evidenced by its larval form) may be the actual closest living relative.

My Amphioxus sketch has the outer skin and muscle on one side "cut away" so that the dorsal nerve cord (darkest line) and the cartilaginous nodocord (directly beneath nerve cord, shaded lighter) can be seen. Also visible are the v-shaped muscle segments (myomeres), gills slits, and gonads (the squarish-round things in a row beneath the gill slits).

The brittle star belongs to the phylum Echinodermata (meaning "spiny skin"), which includes animals such as sea stars (A.K.A."starfish"), basket stars, sea cucumbers, sand dollars, and sea urchins. A brittle star might look like a sea star, but brittle stars and sea stars belong to two different classes. Brittle stars belong to the class Ophiuroidea, while true sea stars belong to the class Asteroidea. So what's the difference? For one thing, they have different means of locomotion. Brittle stars move rapidly by undulating their whole arms in a snake-like motion, while sea stars move slowly using many tiny tube feet on on the underside of their arms. The brittle star's arms have an internal skeleton made up of many vertebral ossicles, which are tiny plates made of calcium carbonate; and the central disk, to which these arms are attached, is clearly defined.

You can see the tube feet, which look like tiny suction cups, on this sea star here:

And here is a picture of a brittle star: