The royal natural history . l methods ; althoughthis, too, presents peculiarities. Justas a butterfly does not developdirectly from the egg, but passesthrough the intermediate larval stage of the caterpillar, out of whose chrysalis thebutterfly springs, so the sea-urchin or the star-fish egg gives rise to a larval form,in whose body, as it were, the mature form is developed. The particular shape ofthe larva varies in the different classes of echinoderms: but the differences are notessential, and it is clear that all the larval forms are modifications from one primitivetype. The changes passed
The royal natural history . l methods ; althoughthis, too, presents peculiarities. Justas a butterfly does not developdirectly from the egg, but passesthrough the intermediate larval stage of the caterpillar, out of whose chrysalis thebutterfly springs, so the sea-urchin or the star-fish egg gives rise to a larval form,in whose body, as it were, the mature form is developed. The particular shape ofthe larva varies in the different classes of echinoderms: but the differences are notessential, and it is clear that all the larval forms are modifications from one primitivetype. The changes passed through in the development of the common sea-urchin{Strong ylocent rot us drcebachiensis) are depicted in the illustration on p. 318, inwhich the drawings are very greatly magnified. The fertilised egg divides and subdivides until a round ball of cells is is then pushed in at one end, as one might push in a soft indiarubber ball,so that there is formed a little sac with a double wall to it (stages 1. 2). Stage 3. green snake-star (enlarged 5 times). 316 ECHINODERMS. shows this in outline as though transparent; and one sees the opening a turneddownwards, and the canal d, which foreshadows the intestine. At the upper pole
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
