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THE POPULAR SCIENCE MONTHLY. 



ments and allow itself to roll over into its natural position. But an 

 echinus will never let go its attachments without some urgent reason, 

 seeming to be above all things afraid of being rolled about at the 

 mercy of currents, and therefore it lets itself down almost as slowly as 

 it pulled itself up (Fig. 10). 



Single rays separated from a star-fish crawl as fast as the entire 

 animal, and likewise in a determinate direction. They also crawl up 



Fig. 10. 



perpendicular surfaces, and when inverted right themselves as quickly 

 as do the unmutilated creatures. A segment of an echinus bearing a 

 single row of ambulacral feet, when propped up on its ab-oral pole, 

 (Fig. 11) will right itself after the manner of entire animals (Fig. 12). 

 It, however, experiences more difficulty in doing so, and very often 

 fails to complete the manoeuvre. Such a segment is, of course, analo- 

 gous to a single detached ray of a star-fish ; but on account of the 

 rigid consistence and awkward shape of the segment standing erect 

 instead of lying flat it presents a much more curious appearance in 

 locomotion than does the ray of a star-fish. 



Dr. Romanes reports observations which show conclusively that 

 the whole external surface, not only of the soft and fleshy star-fish, 

 but even of the hard and rigid echinus, is everywhere sensitive to 

 stimulation. This sensitiveness, moreover, is highly delicate. If any 

 part of the external surface of an echinus is lightly touched with the 

 point of a needle, all the feet, spines, and pedicellarise within reach of 

 that part, and even beyond it, immediately close in upon the needle 

 and grasp it tightly. This simultaneous movement of such a little 

 forest of prehensile organs is a very beautiful spectacle to witness. 

 Here we have proof of the function of the pedicellarife. In climbing 

 perpendicular or inclined surfaces of rock covered with waving sea- 

 weeds, it must be of no small advantage to an echinus to be provided 

 on all sides with a multitude of movable stalks bearing forceps, which 

 can instantly seize a passing frond. The frond being thus arrested, 

 the spines come to the assistance of the pedicellarise, and both to- 

 gether hold the sea- weed steady till the ambulacral feet have time to 



