20 COMPOUND HYDROIDS. 
Development of Discophore.—They multiply by eggs 
that are deposited in.the autumn, and are at first spheri- 
cal, quickly changing to a pear-shape, and attaching them- 
: 
~ 
Fic. 19,—Portuguese man-o’-war, 
Physalia Arethusa. 
selves to rocks or weed. Now, 
tentacles appear (Fig. 17, 1), 
varying in number from eight 
to twenty-four, and after eight- 
een months the pear divides. 
off into disks until we have a 
pile of scalloped saucers one 
upon another (2). The high- 
est one dies, while all the rest 
break off and swim away, lit- 
tle jelly-fishes, that in time 
grow into the gigantic Cyanea 
or others, as the case may be. 
VaLuE.—They form the food of 
some whales and fishes. 
Note.—The gigantic Cyanea af- 
fords a home under its curtains for 
numbers of fishes and several crus- 
taceans, while in its mouth-folds 
lives a long, tapering sea-anemone 
(Bicidium parasiticum), that inmany 
cases mimics in color that of its 
protector, 
Order III. Siphonopho- 
ra.—These are the most beau- 
tiful of the Hydroids, and 
are rarely seen in the North. 
They are free-swimming colo- 
nies. The Portuguese man- 
o’-war, or Physalia (Fig. 19), is a mere bubble, seemingly 
of the finest satin, that floats upon the surface. From the 
upper portion rises at will a fluted membrane, colored with 
delicate tints of pink, that is used as 4 sail; from the low- 
