HYDRA AND CCELENTERATES . 
135 
somewhat different. The egg from one of these medusae 
grows into a fixed, more or less hydra-like form which 
at maturity splits horizontally into a number of saucer¬ 
like disks. These finally break away and become the 
large jelly-fish we so commonly see in salt water (Fig. 
114). 
Sea-anemones. In its cylindrical shape, the position 
of its mouth and tentacles, and in possessing thread- 
Fig. 114. —The Origin of a Scyphozoan Jelly-fish (Medusa aurita). 
a , adult; b, c, d , e, f, g , earlier stages. 
cells, the sea-anemone resembles the hydra. The 
larger size of the sea-anemone makes it easy to study 
in localities where it can be used alive. 
In the anemone the hollow body is traversed by 
radiating partitions called mesenteries. These serve 
to increase the area exposed to digested food. There 
is no hypostome present, and the internal cavity is 
reached by a passageway called the gullet. 
Corals . The coral animals resemble the sea-anemone 
in structure. They differ chiefly in the fact that they 
deposit a hard skeleton. In the more common corals 
the skeleton shows septa corresponding to the mesen¬ 
teries in the living animal. These animals live for the 
most part in colonies, the single individuals, or polyps, 
being connected at their bases in such a way that nutri¬ 
ment may pass from one to another. The examination 
