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

FURTHER PATTERNS OF INDIVIDUALIZED ANIMAL LIFE

Fig. 88. Asterina, a little starfish, about life-size.

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In 1 the animal has been turned upside down ; in 2 to 6 it rights itself again, and in 7 it crawls away. In 8 two individuals are browsing over a gelatinous colony of sea-squirts.

The starfish feeds on animal prey—colonial sea-squirts or polyzoa or small worms in the case of Asterina, oysters, mussels, scallops, and so on in the case of some larger forms, other starfishes in the case of the seven-armed Luidia. They have no masticatory apparatus. A few starfishes take their victims whole into their bodies and there digest them ; others hold them by means of their tube-feet and turn their own stomachs inside-out through their mouths on to the prey, digesting it outside the body. Sometimes Asterina may be seen to browse slowly over colonies of seasquirts by means of its everted stomach. —

; The starfish has no internal skeleton, but its integument is made hard and stiff by a network of bony plates and spines. Scattered over the surface of its body are little pincer-like structures, the pedicellarie—tiny groups of two or three toothed jaws mounted on a muscular stalk. By means of these gnashing pincers the starfish cleans its body and defends itself against its enemies ; in one or two instances they have been seen to be used when attacking prey, and sometimes they secrete poison.

The echinoderms are an exclusively marine group. The sausage-shaped trepangs or béches-de-mer are an important luxury food among Eastern peoples, and there exists a

large industry for collecting them from the reefs of California and the Eastern Archipelago. And the spawn of certain sea-urchins is eaten in Mediterranean countries. The chief trouble caused by echinoderms is that some of the larger and more voracious starfishes wreak havoc on oyster beds.

There are five living classes. The starfishes we have already mentioned. The second comprises the _ brittle-stars—surprisingly active animals with round buttonlike discs and very thin, serpentine arms, by lashing movements of which they move along. The third class, the sea-urchins, cake-urchins, and sand-dollars, have no arms. The last two types nourish themselves like earthworms by shovelling mud into their mouths and digesting out any nutritious matter that it may contain, while the prickly urchins are mainly browsers upon seaweed. A seaurchin is a hollow, bony ball bristling with spines, tube-feet, and pedicellariz. Below, on the side on which it crawls, is its mouth, provided with five sharp teeth ; the teeth are moved by an elaborate engine of bones and muscles called ‘“ Aristotle’s lantern.” Above, at the antipodes, so to speak, are the anus, the madreporite, and the openings of the testes or ovaries.

The fourth class ,the holothurians, includes

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