54 METAMORPHOSIS. + > 
in the animal kingdom. In a very general way, the organs 
usually arise from the three primary layers as follows :— 
1, Epipiast.— Epidermis. 
Sense-organs and nervous system. 
Excretory system (partly mesoblast). 
2. Mrsopiast. —Muscular system. 
Skeletal system. 
Blood-vascular system. 
Reproductive system. 
3. Hyposiast.—Alimentary system. 
Fig. 21.—THr METAMORPHOSIS OF THE SILK-worM MOTH. 
The larva or caterpillar spins a cocoon and changes into a pupa (on the stem) 
which later gives rise to the winged moth. Both sexes are shown. 
Metamorphosis.—From the foregoing we see that 
in the course of time any stage in the life-history of a species 
may, to meet a special environment, be especially and cceno- 
genetically developed until a larval stage is produced in 
marked contrast to the adult. 
In some instances this independent evolution of two 
stages in the life of one organism has reached such a 
climax that the adult stage can only be reached by an 
