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

BOOK 2

the cotton-spinners (so called because they exude sticky white threads from the anus when disturbed, in order to entangle and confuse their enemies), sea~puddings, seacucumbers, sea-gherkins, and béches-de-mer, besides a number of curious burrowing, pelagic, and deep-sea forms. They are like sea-urchins that have been softened and pulled out into long sausages, and in the process they have acquired bilateral symmetry, for they creep on one particular side of the body with the mouth in front and the anus behind. Many of them feed like seaurchins, shovelling mud into themselves by means of a ring of spade-like tentacles round the mouth. Others consume minute organisms that float in the sea water ; they wave their sticky tree-like tentacles, entangling the organisms, and then suck them one by one like a small boy sucking jammy fingers.

" The fifth class, the feather-stars and sealilies, are somewhat different from the rest. They have long, branched arms radiating out from a central disc. The feather-stars swim by sinuous movements of their arms, occasionally anchoring themselves for a shorter or longer period by means of a cluster

THE SCIENCE OF LIFE

GEA Pal Ress

of claw-like roots. The sea-lilies are permanently fixed, living at the end of long, jointed stalks.

In addition to these five classes there are curious fossil groups that have become extinct.

We have already stressed the fact that in the majority of cases the symmetry of an echinoderm is radial—instead of having two symmetrical sides to its body as we have, it consists of a number of symmetrical structures radiating out from a central disc. It is curious to note that this is only true of the adult. A baby echinoderm is typically a minute, delicate, transparent creature swimming through the water by means of one or more fringes of cilia that run round its body. Moreover, it is bilaterally symmetrical ; it has front and back ends, right and left sides. But presently a set of rudimentary organs appears on its left side, rudiments that grow at its expense like a tumour and finally absorb the whole of the larva, becoming the radially symmeirical adult. It is as if a lump appeared on the left flank of a child of six or seven and grew at his expense, developing new organs of its own, finally absorbing all his tissues, and giving rise in that way to the

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89. A group of Echinoderms.

On the left are three Sea-gherkins (Cucumaria) catching and swallowing minute organisms. In the centre are two Sea-lilies (Antedon), one swimming and one resting on a stone. Below on the right is a Brittle-star (Ophiothrix), above that

a Starfish (Asterias), and above that again a Sea-urchin (Echinus) .

The characteristic tube-feet can be clearly seen in

several of the animals.

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