GENERAL SURVEY. 139 
of the antithesis between sedentary polypoid and active 
medusoid types which is characteristic of the phylum. The 
delicate iridescent globes, which represent the class 
Ctenophora, illustrate the climax of activity, and have no 
hint of a sedentary phase. : 
In our preliminary survey of the series, we may begin 
with the little fresh-water Hydra (Fig. 68), which is often 
Ne 
Fic. 65.—Wiagram of Ccelenterate structure, endoderm 
darker throughout. 
1. To left, shows longitudinal section of Hydra; to right, of 
sea-anemone, g., gut; g., incipient gullet. 
z. To left, shows cross-section of Hydra; to right, of sea- 
anemone, in the region of the gullet. 
3 To left, shows vertical section of Craspedote Medusoid 
(with velum); to right, of Acraspedote Medusa, with- 
out velum. g., gut; gZ, gullet. 
Note anatomical correspondence of the polypoid and medu- 
soid forms. 
to be found attached to the stems and leaves of water 
plants. The structure here is extremely simple, but the 
simplicity is probably due to degeneration. In favourable 
conditions the polyp may give off daughter buds, which 
remain for a time attached to the parent, and then separate 
as independent polyps. The bud itself, before leaving 
the parent, may also bud, so that three generations are 
present. If we picture this process of gemmation, but with 
