20*2 FUNCTIONS AND INSTINCTS. 



in structure in all the Echinoderms. They are 

 separately retractile, their form is nearly that 

 of a long ampullaceous tube, filled with a subtle 

 fluid ; the elongated tubular part is that which 

 appears without the shell ; the spherical portion 

 remaining within the body : by means of the 

 above fluid, as in the Polypes, 1 the tube is 

 darted forth, or retracted. Belon counted 5000 

 of these suckers in one species. In the sea- 

 urchin star-fish 2 there are twenty rays, and the 

 suckers are so thick as to touch each other. 

 They may probably be of use to them also as 

 organs of prehension to seize their prey. Those 

 of the family to which the Medusa star-fish 

 belongs, move in a different way. The diverging 

 rays are firm and hard, have few spines, and no 

 channel with suckers ; they are used by the 

 animal as legs, and as they are regularly placed 

 it can move in any direction that suits it. To 

 go towards any particular spot, it uses the two 

 rays that are nearest to it, and another that is 

 most distant from it ; the two first curve at their 

 extremity so as to form two hooks, which being 

 applied to the sand drag the body forwards, 

 while the posterior is curved vertically, and per- 

 forms the part of a repelling lever. The suckers, 

 which in this genus issue from the sides of the 

 rays, at the junction of the upper and lower 



3 See above, p. 164. 2 Asterias echinites. 



