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

BOOK 2

been chasing a dark object, follows the black puff ; by the time it has realized its mistake the cuttle-fish has made good its escape. These cephalopods are undoubtedly the most intelligent-looking invertebrates. They have well-developed and grimly expressive eyes, and the subtle coiling movements of their arms and the rapid and very striking colour-changes that they undergo when stalking their prey or when pursued are suggestive of mental accompaniments. Observers who have watched the very elaborate courtship of these animals, or who have noted the voluptuous writhings of the tips of the arms when a hungry octopus devours its prey, find it easy to believe that these creatures can experience a rich variety of intense emotions—though it must be admitted that we have, as yet, no clear evidence of memory

or forethought on the part of a cephalopod.

8 3

Echinoderms : Nature’s Pentagonal Experiment

Most of the animals that we have considered so far bear a certain very crude resemblance to ourselves in the way their bodies are laid out. They have front ends where the chief sense-organs and the mouth are placed, they have hinder ends, they have backs, and bellies on which they crawl. A bivalve or a flatworm offers difficulties ; but for the most part, by stretching one’s imagination just about as far as it will go, one can get a crude idea of what the animals feel like. If, for example, one were covered with close-fitting combinations made of medizval armour, and if the number of one’s limbs were legion, one would feel not unlike a lobster, and if one had no arms or legs and lived in a pitch-dark tank of warm thick soup one would feel not unlike the roundworms to be presently described. But with the phylum Echinodermata things are different ; with a few exceptions they do not know front and back, right and left. They are not bilaterally symmetrical, as we are. ‘They do not “ look before and after.” One cannot imagine oneself a starfish or a sea-urchin however hard one exerts one’s mind.

A starfish consists of a number of armsusually five—radiating out from a central disc—but the so-called ‘“‘ arm” of a starfish is no more like our own arms than a lobster’s “liver” is like our livers—indeed, hollow as it is and containing viscera, it is more like a belly than an arm. Its mouth is exactly in the middle of that disc, on the lower side on which it crawls ; its anus is in the middle of

134

THE SCIENCE OF LIFE

CHAPTER 3

the upper surface. Its mouth opens into a loose, baggy stomach ; that leads into a short intestine ; from the intestine ten long branches run off, two into each armbranches not unlike the liver of a lobster, for the finer food-particles go into them to be digested and absorbed. If we turned a starfish on to its back we should notice five broad grooves radiating away from its mouth, and one running along the whole length of each arm ; in each groove there is a serried double or quadruple row of little tubes that end in suckers. The tubes are muscular. They can be stretched out or drawn into the grooves or waved from side to side, and by movements of these hundreds of “ tubefeet,” gripping the ground by means of their suckers, the starfish crawls along. Inside its body it has an elaborate system of pipes and reservoirs containing a watery fluid and opening to the exterior through a porous, sieve-like plate, the madreporite, that is placed near the anus; the tube-feet are stretched out by forcing water into them from this system, and drawn in by allowing it to flow back. This ‘‘ water-vascular system’ is quite unlike anything found elsewhere ; it is one of the many unique and characteristic features of these echinoderms. Fig. 88 shows a very common little starfish of our shores, Asterina, crawling along. One of its five arms—it does not matter which—is a temporary head ; it is curved upwards so that the little scarlet eye on the tip may (albeit very dimly) perceive coming dangers, and the two or three end pairs of tube-feet are extended in the water like the antenne of an insect. The other arms are subordinate ; they are simply walking, and not sitting up and taking notice. If we tap the front end of the leading arm, or if for any other reason it occurs to the animal that the direction in which it is proceeding is not desirable, that arm ceases to act as a head and becomes passive ; one of the other arms holds up its eye-spot and waves its tentacles like antenne, and the animal follows the new head in a new direction. It will be noted that at any one time the many hundreds of tube-feet are all pulling together, some working in a direction parallel with the groove in which they sit, some, in other arms, working across their grooves at various appropriate angles. How much more applicable to the starfish than to the centipede is the celebrated epigram : “The centipede was happy—quite !

Until the toad in fun

‘This raised her doubts to such a pitch,

She fell exhausted in the ditch,

Not knowing how to run!”