156 PHYLUM C@ELENTERA. 
processes the embryo elongates, the outer cells become ciliated, and 
the mouth closes. Thus the embryo becomes a free-swimming oval 
planula, 
After a short period of free life, this planula settles down on a 
stone or seaweed, attaching itself by the pole where the mouth formerly 
opened, Ata very early stage the mesoglcea appears between the two 
layers. At the free pole an ectodermic invagination next occurs, an 
opening breaks through at its lower end, and thus a gullet lined with 
ectoderm is formed, which hangs freely in the general cavity. During 
this process there are formed first two and then four diverticula of the 
Fic. 75.—Diagram of life history of Aure/ia.—After 
Haeckel. 
1. Free-swimming embryo ; 2-6, various stages of Hydra-tuhba ; 
7, 8, Strobila stage; 9, liberation of Ephyre; ro, 11, 
growth of Ephyra into Meduse. 
general cavity, which are arranged round the gullet above, and open 
freely into the digestive cavity below. In the gullet region these are 
separated by broad septa, which are continued into the lower region of 
the body as four interradial ridges or teeniolee. The tentacles bud out 
from the region of the mouth, the first four corresponding in position to 
the four pouches. Interradially above the four septa, four narrow 
funnel-shaped invaginations arise ; these are produced by the ingrowth 
of ectoderm, which then forms the muscle fibres which run down the 
teeniolze (contrast the ezdodermic muscles of Anthozoa). In contrasting 
this development with that of the hydroid polyp, Goette specially 
