TABLE SHOWING THE CLASSIFICATION OF THE POLYZOA
{ ECTOPROCTA, with excretory aperture outside { the ring of tentacles, _e.g._, _Flustra_. =POLYZOA.= { { ENDOPROCTA with excretory aperture inside { the ring of tentacles.
The Polyzoa include freshwater as well as marine forms. They have a free-swimming larva, which becomes fixed after a time, and gives rise to the adult Colonial forms. The zooids of the latter have each an independent head with a crown of tentacles, called the Lophophore (Crest-carrier); but the fixed ends of their bodies communicate with one another. The hard covering of the colony, which retains its form after the animal is dead, is a kind of hardened skin: the apparent "cells" are the openings through which the individual zooids protrude themselves. Sometimes certain of the zooids undergo modification for special purposes: in this way are formed the "avicularia," snapping appendages, probably defensive in purpose, so called because they open and shut like a bird's beak. There are two divisions of the Polyzoa, the Ectoprocta and the Endoprocta. Among the latter there is found a form which is not colonial.
_Phoronis_, a curious worm-like animal, which has a larval form called _Actinotrocha_ is sometimes placed in classification near the Polyzoa, which it resembles in possessing a crown of tentacles (Lophophore).
Everybody knows the Star-fish and many people know the Sea-Urchin. An "urchin" is not a name for a naughty little boy, but the French (_oursin_) for a hedgehog. A Sea-Urchin is therefore a "Sea-Hedgehog," a name very appropriate for a creature armed with prickles. The Greek word _echinos_ also means a hedgehog, so that the long name given to the group means simply hedgehog-skinned. The prickles attain their maximum in the Sea-Urchin, but they are well represented in the Star-fish, while in the Sea-cucumber the general tendency to "prickliness" is much reduced, and represented only by "spicules" (needles) of shelly stuff underneath the skin of the animal.
The largest and the most beautiful of the Sea-Urchins of the English coast is known as the Purple-tipped Sea-Urchin, on account of the beautiful colour of the spines. It lives on rocky coasts, and during very low tides may be seen at home, although it usually takes care not to stray above the water-line. It is a shelly ball with a flat base; its surface is covered with long spines. Its mouth, which is in the centre of the base, shows five wicked-looking teeth peeping out. The shell is pierced by what look like hundreds of minute pin-holes, arranged in a complicated pattern; these are the holes through which it pokes its feet, which greatly resemble those of a Star-fish, being white suckers with a disc at the end. When thrown out to their full length they are, however, much longer than those of the Starfish, for they are naturally obliged to be thrown out to a distance longer than the length of the animal's own prickles. When moored by all its feet, extended from all sides of the shelly ball, the animal presents a curious and pretty sight. Large specimens are almost as big as a child's head, but smaller ones are more common. There is a considerable range of variation in colour; not only are various shades of purple found, but also purplish-red and red. The spines are mounted on something resembling a ball and socket joint, with a ring-shaped pad, so that they have a wide range of movement; if any of the spines are touched they are immediately set back over a considerable part of the neighbouring surface.