centre of each of which is a ball, upon which a spine works on a ball and socket joint.  These spines are used for protection, and when large they can be used for locomotion.  But the real means of locomotion are five double rows of water-tube feet, working by suction, by which they withdraw the water inside a receptacle in the shell, thereby forming a vacuum; starfishes do the same.  We found a species of sea-urchin which had such large spines that they practically formed bars; the spines were twice as long as the sea-urchin and shaped just like oars, being even fluted.  A lobster grows by discarding his suit, hiding and getting another, growing meanwhile.  A snail or an oyster retains his original shell, and adds to it in layers all the way down, increasing one edge.  But our sea-urchin grows by an increment of calcareous matter all round the outside of each plate.  As the animal grows the plates get bigger.

There was a sea-cucumber which nurses its young, having a brood cavity which is really formed out of the mouth:  this is a peculiarity of a new Antarctic genus found first on the Discovery.  It has the most complex water-tubes, which it uses as legs, and a few limy rods in its soft skin instead of the bony calcareous plates of sea-urchins and starfish.  After them came the feather-stars, a relic of the old crinoids which used to flourish in the carboniferous period, examples of which can be found in the Derbyshire limestone; and there were thousands of brittle-stars, like beautiful wheels of which the hubs and spokes remained, but not the circumference.  These spokes or legs are muscular, sensory and locomotive; they differ from the starfishes in that they have no digestive glands in their legs, and from the feather-stars in that they do not use their legs to waft food into their mouths.  Once upon a time they had a stalk and were anchored to a rock, and there are still very rare old stalked echinoderms living in the sea.  This apparently geological thing was found by Wyville Thomson in 1868 still living in the seas to the north of Scotland, and this find started the Challenger Expedition for deep-sea soundings in 1872.  But the Challenger brought back little in this line.  Most of the species we found were peculiar to the Antarctic.

There were Polychaete worms by the hundred, showing the protrusable mouth, which is shoved into the mud and then brought back into the body, and the bristles on the highly developed projections which act as legs, by which they get about the mud.  These beasts have apparently given rise to the Arthropods.  In a modified and later form they had taken to living in a tube, both for protection and because they found that they could not go through the mud, which had become too viscous for them.  So they stand up in a tube and collect the sediment which is falling by means of tentacles.  They spread from one locality to another by going through a plankton embryonic stage in their youth.  They may be compared to the mason worms, which also build tubes.