112 THE WORLD OF THE SEA. 
seen, utterly different from their parents. They must undergo many 
metamorphoses before they reach their perfect state. These meta- 
morphoses are just the reverse of those through which insects pass. 
The chrysalis lies immovable; but finally becomes a butterfly. 
With the coral, the larva has the power of locomotion, whereas 
the full-developed polype is fixed. There is not perhaps in nature 
a single law whose reverse is not also found in operation. 
\ MAGNIFIED SECTION OF THE STEM OF THE ISIS, 
The true corals are divided into the J/el/t@a and the /szde. 
These latter have their branches articulated, and their polypes 
possess six tentacule instead of eight ; these tentaculz being entire 
and not separated. In the former, the stems are gnarled here and 
there at intervals, and are covered with a firm and resisting cortex. 
In the latter, they are articulated, and the crust is horny and 
chippy. The material of the Melitea is stony and homogeneous ; 
while that of the Iside is composed of two substances, the 
compressed parts are horny and black, the articulations being 
calcareous and striated. The stem of the Iside is often sold for 
