The science of life : fully illustrated in tone and line and including many diagrams

LESS INDIVIDUALIZED ANIMALS

of the tissues and a decrease in their power of developing independently and giving rise to new parts. In ourselves, the healing of a wound, the formation of new skin to seal a cut, of new fibres when a muscle is torn, of new bone when an arm or a leg is brokenthese are the only representatives, the physiological homologues, so to speak, of the great power of reconstitution possessed by the sea-squirt.

Among invertebrates, the lobster is at about the level of a newt in this matter of reproductive power; it can part company with a limb in order to save itself, and it can slowly grow a new one. An octopus, too, can grow a new arm. And in the starfishes the power of reconstitution is amazingly well developed. If a starfish be cut in halves, each half will sprout new arms and become a whole starfish—if a little of the disc is attached a single isolated arm can grow the rest again. ‘There are species of starfish which normally use this power, like the sea-squirts, for reproduction. After living harmoniously together for some time the arms seem to fall out and to group themselves into two parties; one party walks as hard as it can in one direction and the other party pulls equally hard in the other, so that the central disc with mouth, stomach, anus, and the rest is split right across. Finally, each of the two halves grows the missing parts and becomes a complete starfish. Fishermen, infuriated by the quantities of starfish that appear in their nets and by the havoc wrought by these animals on oyster-beds, often chop them in halves and fling them back into the sea; but since each half then proceeds to develop into a whole © starfish, the fishermen are increasing

Fig. 96. A “Comet Star-

Jish °—a single starfish arm,

accidentally cut off, is growing the missing parts.

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rather than reducing the numbers of the species.

In the segmented worms we find various methods of asexual reproduction. In the little marine worm Autolytus cornutus, new individuals are grown at the hinder end of the body, so that there may be a chain of from two to at least forty individuals, each with a head of its own, but stuck together head to tail by a mortar of living tissue until such time as one of the hinder ones becomes rebellious, refuses to follow its leader, and tears itself away. Moreover, in this species we have an example of the phenomenon known as“ alternation of generations.” The fertilized eggs develop into sexless individuals which swim about sprouting new individuals until they die—but the individuals thus sprouted are male and female (and differ in a number of external characters from the sexless generations) ;_ they do not reproduce themselves asexually but produce fertilized eggs—and so the cycle begins again. In another kind of worm, Syllts ramosa, which is found inside the cavities of sponges from the deep sea, new individuals are sprouted off as side-branches from the flanks of the old, so that a web of worms is produced from which the sexual individuals snap themselves off and escape.

Finally, similar but even more striking phenomena are seen in flatworms. In most species of Planaria (but not In all) the creature creeps about as we have described, but while it does so there is disaffection, so to speak, in its end; as the animal becomes fully grown the latter half reorganizes itself into a new individual, and when it is ready it suddenly rebels and firmly grips the ground like a resolute donkey, refusing to be pulled along by the front any longer. There is a frantic tug-of-war and a final split; the front end crawls away with an appearance of extreme indifference to grow a new and

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Fig.g7. The hinder half of this marine worm has developed a head of its own and will soon break away.