126 THE STORY OF ANIMAL LIFE 



float, as it were, in the same stratum of water 

 without sinking further down. This creature, on 

 the contrary, has apparently feared lest it should 

 move too slowly when it moves in a vertical 

 direction, and it presents us with an arrangement 

 by means of which its sinking through water is 

 facilitated. Water will pass readily through the 

 five holes as the animal goes either up or down, 

 and the resistance of the whole flat area to the 

 water is thus reduced and vertical movement ren- 

 dered more easy. Thus, by one and the same 

 contrivance, the animal has lessened its weight 

 when lying quiet, and diminished the resistance it 

 meets with when it moves. The distribution of 

 the holes, moreover, is such as to regulate the 

 animal's position in sinking, and to prevent it 

 from falling " headlong." For although the crea- 

 ture has, strictly speaking, no "head," yet the end 

 nearest the mouth is the thickest and heaviest 

 part of the "cake," and would naturally tend 

 downwards. This tendency is counteracted by 

 the fact that the thicker end is unperforated, 

 while the thinner and lighter end has a large 

 central hole to diminish its resistance and enable 

 it to sink more rapidly. 



Adapted for living in sand rather than on 

 rocks, but not so extreme in the peculiarity of 

 their form as the Shield-Urchins, are the Heart- 

 Urchins, already referred to, shaggy-looking 

 creatures whose fine yellowish-white spines give 

 them almost the appearance of being clothed 

 with fur. The excretory aperture is at the nar- 

 row end of the " heart," and the mouth at one 

 side of the lower surface towards the wide end. 

 The complicated apparatus of teeth found in 

 other Sea-Urchins is absent in these. They arc 



