WHEN the tide goes out on a sandy shore it leaves little behind to reveal the secrets of the sea’s inhabitants. There are no rock-pools, as on the more rugged parts of the coast, rich with a profusion of sea-anemones, sponges, shrimps
and crabs; but disappointing as a sandy shore may be there are yet some things to be seen by those interested. Not least among these is the little sand-mason worm, a creature of lowly station and meagre abilities yet which, nevertheless, is able to build itself a beautiful tube of sand, small pebbles and fragments of shell. We can see these tubes at low tide, lying limply on the damp sand, but we find no trace of the owner for it has disappeared below to await the return of the tide.
If we could watch the sand-mason worm at work we should see a rather wonderful sight. This worm has a crown of tentacles surrounding the head which can be extended for a long distance, and as they grope about they grasp the sand-grains or the pieces of shell which come within their reach. These then pass down the length of the tentacles as though moved by invisible hands until they reach the mouth; actually they are passing along a groove in the tentacle specially provided for the purpose, and propelled by fine protoplasmic-hairs lining the grooves. Each particle so obtained is taken into the mouth, moistened with an adhesive fluid, and then ejected and placed into position by the lips. So grain by grain the tube is built up, a monument to tireless energy and patience.
There are many difficulties in the way of seeing the sand-mason worm actually at work, but it is fairly simple to see a similar process being carried on, provided a microscope is available, by a small animal living in freshwater. This is one of the Wheel-animalcules, or Rotifers, known as Melicerta, which grows in ponds and streams on the leaves of water-weeds. When fully extended the body of Melicerta is less than an eighth of an inch long, cylindrical, and bearing at the summit a four-lobed head with a mouth set in the centre. The margins of the lobes are fringed with protoplasmic hairs in a constant state of rhythmic motion giving the effect of four continuously revolving wheels. Normally the body is enclosed within an upright tube built up of minute spheres of a dark material arranged in a remarkably regular series. When feeding, the animal’s head is protruded beyond the tube, but at rest the whole body is drawn inside.
To see how this exquisitely formed tube is made we have only to wait for Melicerta to protrude its head from the tube, when the action of the ‘wheels ‘sets up a continuous stream of particles into the mouth. The transparent nature of the animal enables us to see the further fate of the particles.
Some are passed on to the stomach, others, the indigestible fragments, pass into a small cavity within the head and are there revolved until they are compacted into a small, round pellet. This is then passed back to the lips and, with a convulsive jerk of the head, the lips are protruded and the pellet placed in position on the wall of the tube. Regularly and with never-failing precision, the pellets are formed and placed into position, the wall of the tube being built up, a row at a time, as though composed of microscopic beads.