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THE POPULAR SCIENCE MONTHLY. 



the animal while walking be turned half round, it will continue its 

 movements as before, and hence will proceed in a direction opposite 

 to its former one. When at rest, some of the feet are used as anchors, 

 and others protruded as feelers. 



All species of the Echinodermata, when turned upon their backs, 

 are able to right themselves. The brittle-stars can easily perform the 





Fig. 7. Natural righting Movements of Common Star-pish. 



needful manoeuvre by wriggling some of their snake-like arms under 

 the inverted disk, and heaving the whole body over by the mere mus- 

 cularity of these members. The common star-fish, however, experi- 

 ences more difficulty, and executes the manoeuvre mainly by means of 

 its suckers. It twists round the tip of one or more of its rays until 

 the feet there situated are able to get a firm hold of the floor (Fig. 7, 



t ffll l t l l ll l l llllHllllll l ll ll llll ll l ll llll lll lll l l H^ 



Fig. 8. 



a), then, by successive action of the feet further back in the series, the 

 whole ray is twisted round (b), so that the under surface of the end is 

 applied flat against the floor (c). The semi-turn or spiral then travels 



