The royal natural history . tube-feet. If one slightlywounds the animal when thus expanded, the larger spines immediately stiffen ontheir tubercles, while all the smaller spines depress themselves, each over theorgan that it is destined to protect. Though the tube-feet may not be used forlocomotion, they are put to another useful purpose. If a Strongylocentrotus beplaced in a tank with some dead shells or similar objects, it will raise them on toits back, and hold them there by means of the tube-feet, as a kind of sea-urchins cover themselves all over in this way with bits of
The royal natural history . tube-feet. If one slightlywounds the animal when thus expanded, the larger spines immediately stiffen ontheir tubercles, while all the smaller spines depress themselves, each over theorgan that it is destined to protect. Though the tube-feet may not be used forlocomotion, they are put to another useful purpose. If a Strongylocentrotus beplaced in a tank with some dead shells or similar objects, it will raise them on toits back, and hold them there by means of the tube-feet, as a kind of sea-urchins cover themselves all over in this way with bits of seaweed, shell,and small pebbles, and so move about unobserved. Other sea-urchins do not move SEA-URCHINS. 3i from place to place, but always stay in one spot, where they are generally foundliving in a hole. Sometimes the hole may have been there before the sea-urchins ?sometimes may have been formed by the growth of calcareous alo-ad around thesea-urchin; but sometimes the urchin itself has bored the hole. This is accom-. phial-shaped fourtalesia ; test with spines removed (enlarged 4 times). plished not by any acid secretion,—for on the west coast of Africa an Echinornetrahas been found boring into an augite lava,—but by the continuous movement of theteeth and spines. The common Strongylocentrotus is a well-known example of aboring sea-urchin. When the waves wash up against the urchin it sets its spinesrigidly against the sides of its hole and so holds fast. Although most of the sea-urchins have a rigid test, yet there are some inwhich the plates are only loosely joined together, so that the test is flexible.
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
