Sea Urchin Anatomy

Sea urchins control their tube feet via their water vascular systems. Tube feet exist on the outside of the sea urchin, and are how sea urchins grab and bring food towards their mouth, and move along the sea floor. The ring canal is central to this system, with five radial canals branching out from this central piece along inside of the test. These radial canals supply water to the tube feet, enabling motor functions. The water vascular system works like a hydraulic system, where ampullae act like muscles to enable movement by manipulating pressurized water within the system. This system is universal of echinoderms, a phylum which also includes sea stars, brittle stars, and sea cucumbers.

Sea urchins in the subclass Euechinoidea, a subclass encompassing most extant urchins, have epithelial tissue - a skin-like lipid membrane, surrounding their spines. This tissue prevents organisms like sponges and algae from growing on the spines, which is why these organisms are absent growing on most species of urchins. Conversely, urchins of the subclass Cidaroidea uniquely lack epithelial tissue, which enables symbiotic sponges and algae to grow on these particular species of urchins (David and Saucède 2015).