SEA-UBCHIN. 177 



diameter, the walls of which shall be scarcely thicker 

 than a wafer, formed of unyielding limestone, yet fitted 

 to hold the soft tender parts of an animal, which quite 

 fill the concavity at all ages. But in infancy the animal 

 (and, of course, its box, as this must be full) is not so 

 big as a pea ; and it has to grow till it attain its adult 

 dimensions. The box is never to be cast off, and re- 

 placed by a new one; the same box must hold the 

 infant and the veteran Urchin. The limestone, not being 

 a living tissue, but an inert earth, can grow only by 

 being deposited. Now the vascular tissues are within, 

 and the particles they deposit must be on the interior 

 walls. This would indeed augment the amount of 

 limestone in the box, but it would be at the expense of 

 the contained space. The thicker the walls, if thick- 

 ened from within, the less room in the cavity ; but what 

 is wanted is more room, ever more, and more. The 

 growing animal feels its tissues swelling day by day, by 

 the assimilation of food, and its cry is, " Give me space ! 

 a larger house, or else I die." 



How is this problem solved ? Ah ! there is no diffi- 

 culty. The inexhaustible wisdom of Jehovah the 

 Creator has invented a beautiful contrivance for the 

 emergency. The box is not made in one piece, nor in 

 ten, nor in a hundred ; six hundred distinct pieces go 

 to make up the hollow case ; all so accurately fitted 

 M 



