PROBABLY no other animals present such bizarre forms as those collectively grouped under the heading of the Echinoderms, and including the starfish, sea-urchin and sea-cucumber. Many of the sea-anemones and other polyps of a like nature have quaint forms, but they are on the whole so reminiscent of plants that they do not appear unfamiliar to us. Were we, however, to be suddenly faced with drawings of a starfish or sea-urchin, without having previously seen or heard about these animals, it is probable that we should regard the pictures as figments of an artist’s imagination. The star-like build of the starfish, with no apparent head or tail, and little externally to suggest the possibility of its being an animal, or even a living thing, except the ability to move about, is surpassed for quaintness only by the box-like body, covered with a closely-set crop of spines, of the sea-urchin. It is obvious from the attitudes of deep concentration maintained by visitors to the seashore as they stoop watching a stranded starfish that many questions and speculations are
passing through their minds as to what these strange beings are and how they live.
The organisation of a starfish’s body is really quite simple. The central disc contains the essential organs, the stomach, intestine and so on, and the arms are purely organs of locomotion or used in the capture of food. The mouth is on the under surface of the central disc and can be seen by turning the creature over on to its back. In animals with so unusual an exterior we should naturally expect to find some unusual anatomical features, and in this we shall not be disappointed.
Once the starfish is turned on to its back, we shall have our attention drawn to the rows of ackers, hundreds in each row, running down the centre of each of the radiating arms. It is in the possession of these more than in any other respect that the starfish, and other Echinoderms, differ so markedly from all other animals. These suckers are known as tube-feet and are connected up with a series of tubes within the body, forming a hydraulic system. By throwing the tube-feet well out and fastening them on to the surface of a rock, the starfish is able to pull itself along, the tube-feet working in unison. While some are maintaining a hold others are being retraced to pull the body along and others are being thrown out to obtain a fresh hold farther ahead.
HOW A STARFISH TRAVELS AND COLLECTS ITS MEALS WHEN a starfish is lying on its back the tube-feet wave about in different directions, apparently quite independent of each other, but when the animal is right side up and moving along, the feet move with one accord and in one direction, for the reason that they are all connected by a continuous system of tubes. Their efficiency as feet depends on this that they can be converted into suckers when necessary. When the terminal discs, which are hollow also, touch the surface of a rock they flatten against it and the floor of each is raised by the manipulation of slender muscles operating inside the foot. Thus a vacuum is created between the disc and the surface of the rock. The combined pull of the suckers is sufficiently strong to enable the starfish to cling to a rock in the roughest of seas, or to climb up the face of a vertical obstruction.
Another use of the tube-feet, quite an important one, is in pulling apart the shells of mussels and other shellfish on which the starfish feeds. The two shells of a mussel are held
together by a powerful muscle running from the inside of one shell, through the body of the animal, to the opposite shell, and the strength of this muscle may be gauged by the amount of prising open that is necessary before the morsel inside may be extracted. The starfish, having nothing with which to prise, has another method of accomplishing this end. Wrapping its arms round the shell, it takes a firm hold with its tube-feet and begins to pull until it has pulled the shells apart. Then it everts its stomach, wraps it around the body of the mussel and proceeds to digest it.
Not only are the tube-feet used as organs of locomotion and as weapons of offen-j, but they also serve as respiratory organs. Through them oxygen is absorbed from the sea-water, but in spite of their numbers the amount of oxygen they can absorb is insufficient to meet the requirements of the starfish; consequently some means must be found of augmenting the supply. The starfish’s skin offers few possibilities in this direction since it is studded with numerous plates of lime and constitutes an effective barrier to the passage of oxygen. An interesting compromise is, however, effected. Between the limy plates the skin grows out at intervals in groups of small, finger-shaped folds all over the surface of the body, and through these oxygen can be absorbed.
Having to adopt a second-best alternative, however, something more than the bare skin is necessary to ensure complete respiration, and an ingenious device is therefore used. These minute folds of skin are coated both inside and out with extremely fine protoplasmic hairs, constantly waving in one direction. Those on the inside keep the fluid of the body moving, so that as soon as one lot is saturated with oxygen it is passed on and another lot devoid of oxygen can take its place. The sea-water, also, is kept in constant movement by the protoplasmic hairs on the outside so that as soon as one lot of sea-water is deprived of its oxygen it is driven away and a fresh supply takes its place. By this combined action the maximum amount of respiration can take place.