Family Of Fives

Press, Volume CVIII, Issue 31730, 13 July 1968, Page 5

Family Of Fives

(Contributed by the Canterbury Museum)

female organs; of grills—in those that have gills; and five lots of digestive organs, and they all radiate out from the centre of the animal. Fortunately they are able to manage with one mouth and stomach, which is in the middle of the animal with the mouth on the underside. Those of you who have collected sea stars, or who go skin diving for the sea urchin or kina, perhaps to eat, or who have been given a dried sea star to keep as an ornament, must have thought that they look as though some invisible mathematician or design artist bad planned quite precisely how they were going to be in nature. Geometrical They are masterpieces of Nature’s geometry, with axes, radial lines, and segments permanently marked on every part of their body. However, there are as usual, in animal groups, kinds that don’t conform to the rule, and break away from the picture of perfect symmetry or pattern of fives. One of the commonest sea stars has six arms, three longer than the others, and a giant spiny star that the trawlers sometimes bring in has not five times two, or ten arms—but eleven. In most, there is some sign that the animal has not obeyed the perfect pattern. One finds sea stars and brittle stars with arms of unequal length, but often this is because the creature has at one time or other lost one of its arms, and the new one that regenerates is for a long time, very much smaller. You do not have to look very hard to see that all these animals are usually heavily armour-plated and often spiny. Their group name of Echinodermata is derived from Greek words that mean spiny-skinned. The plates and spines are actually covered by a very thin skin and almost all the heavy armour is made of crystals of calcite (calcium carbonate). Every plate of the hundreds on a sea urchin's shell and each of the spines, as well as almost all the skeleton pieces of a starfish,

Among the curiosities of Nature, seastars, brittle-stars and sea-urchins attract our attention because of their curious shapes, strangely different from what we think of as truly animal. They have no obvious head, nor back nor front, right side or left; ail we can discern is a side facing up and a side facing down upon the rock or sand that the creature sits upon. The general impression is of an animal that is star shaped or, as in the sea urchin, roughly circular. This is the key to their essential nature, for many years ago sea stars, brittle stars and sea urchins (along with the very different sea anemones) were placed in a group called Radiates, because of their “radial symmetry." If you think for a moment of an imaginary rod running through the very centre of a sea star, for instance, see how all the parts of the body —like the arms—actually radiate out from that invisible central rod, and how each arm is more or less identical with the others. This is what radial symmetry means. And if you chance to find a sea urchin without its spines, you’ll see that the hard shell is divided into sections which are just like the arms of the sea star but all joined very tightly together. Common Pattern These sea creatures are not only radially symmetrical, but they also can be called a family of fives, where to have five bits of everything, or so many times five everything is the fashion and the done thing. Most sea stars and brittle stars have five arms. Most sea urchins are not perfectly round, but five-sided or pentagonal, with five main sections in their shells. The pattern of fives is also carried out in their inner organs. There are five lots of nerves; of “blood vessels”; of male or

is made of a single caleite crystal—a most unusual thing in the animal world. Living In Groups All the sea stars, brittle stars and sea urchins (together with two other sorts of echinoderm—with the unlikely sounding names of sea cucumbers and sea lilies) live in the sea. They are usually rather social animals, preferring to live together in groups. This is most important at breeding time, for otherwise the female sea star might have little chance of her eggs developing into more sea stars to carry on the species.

The female simply discharges all her eggs into the open sea, and it appears that the presence of the eggs in the water stimulates the male to release thousands of sperms into the water, which wriggle towards the eggs and fertilise them. Once they are fertilised, the egg develops into a larval stage that does not look a bit like the adult at all.

It is a very fragile, transparent, fairy-like creature, no more than a millimetre long, with curious lobes And vibrating bands of little hairs that propel it through the water. What is most interesting about the larva is that one can recognise a left and a right side, that is, it is bilaterally symmetrical at this stage, not radially symmetrical like the adult. It lives in the ocean plankton for up to four weeks, though some kinds may live for a much shorter period. Ultimately it undergoes drastic changes in its shape and, sinking to the bottom, becomes a miniature sea star (or brittle star or sea urchin, whatever the case may be). There is much we have not yet said about these spinyskinned creatures. How do they move? How do they feed? That is for next week.— M.M.D. and J.T.D. The illustration shows the minute larvae of sea urchin, brittle star and sea star (very greatly enlarged) at the top of the picture, and the adults below.