The Echinoidea (sea urchins, sand dollars, etc.) are a Class in the Echinodermata Phylum. 950 living species have been described, but over 5000 fossil species have been discovered so far.
Echinus esculentus (the edible sea-urchin, below) is the largest and most common species found in U. K. waters, and can be found between the lower shoreline and as deep as 1000 m.
In sea urchins the five arms of the typical Echinoderm have been incorporated into the body (see Echinus esculentus, (above) and the ossicles form a rigid test with pedicellariae and moveable spines which they use for walking. They also have tube feet that aid the spines in locomotion.
The mouth is on the lower surface and has a distinctive chewing apparatus called Aristotle's lantern. It is made up of five large and several small plates, and can be pushed through the mouth. The anus is on the upper surface.
Some in this class have become secondarily bilateral (see the sand dollars), and there is a general movement of the anus to the posterior end and the mouth to the anterior end.
Echinoidea are found on the bottom of all seas at all depths. Urchins tend to prefer rocky or hard surfaces, whereas sand dollars prefer to burrow in sand.
Reproduction. The sexes are separate and fertilization is external.
Above is Paracentrotus lividus, the purple sea urchin. It is found in the N. E. Atlantic and the Mediterranean on the lower shore in rock pools and down to a depth of 3 m or more.
It uses its spines to burrow into soft rocks, often limestone, as the depression this creates provides protection from wave action and desiccation at low tide. The depression or burrows increase in size as the urchin grows.
It can reach up to 7 cm in diameter, and is considered a delicacy in some Mediterranean countries. It moves around 4 m per day in an erratic path to graze on algae.
Prionocidaris baculosa, above, is found in the Red Sea and the Indo-Pacific Ocean at depths down to 200 m, but more usually 40 - 140 m deep. If you look closely you can see that its spines are barbed their whole length making them very painful and difficult to remove if they pierce flesh, however as the end of the spine is not very sharp they are usually not much of a danger. Fully grown individuals can reach 5 - 8 cm in diameter excluding the spines.
Prionocidaris baculosa hides during the day in nooks and crannies, and emerges at night to feed. It eats almost anything including algae, sponges, debris and carrion.
The Pencil urchin, Phyllacantha imperialis, above, can be found in the Indo-Pacific and Red Sea. The shell is usually purple or dark brown with thick, striated spines that are often covered in algae. The spines are blunt-tipped. It can be found as deep as 70 metres, but is more often found in shallower waters.
The Pencil urchin hides during the day. It emerges at night to graze on the substrate eating algae, carrion, or almost anything.
The Slate pencil urchin, below, is found in the Indo-Pacific. It is usually a brown colour, and is active mainly at night when it feeds on algae.
Colobocentrotus pedifer also know as Podophora pedifer, above, can be found in the Indo-pacific. It lives on the shore, and withstands buffeting by waves by having its upper spines form a smooth cover, almost like a shield. On its undersurface its mass of tubular feet help it cling to the rocks.