118 THE WORLD OF THE SEA. 
the axis. Their under edge is divided into layers, which are 
covered with little, hard, white needles, of a limy substance, which 
easily break. These layers contain the polypes. They are near 
THE THORNY SEA-PEN. 
to each other, placed obliquely, and their surfaces are traced with 
lines. They have the form of a purse divided into two portions, 
in one of which is the mouth, bordered by its tentacles, and the 
POLYPES OF THE THORNY SEA-PEN. 
other contains the organs of digestion and the ovary sac. In fact 
these polypes resemble flowers, but flowers which quiver with 
sensibility, and are yet incomplete, which possess a will, and yet 
it is limited. : 
