46 ANIMAL LIFE 
stance. The bodies of all jelly-fishes are soft and gelatinous, 
the body substance containing hardly one per cent of solid 
matter. It is mostly water. Many jelly-fishes are beauti- 
fully and strikingly colored, and as they swim slowly about 
near the surface of the ocean, lazily opening and shutting 
their iridescent, umbrella-like bodies, they are among the 
most beautiful of marine organisms. When one of the 
jelly-fishes is taken from the water, however, it quickly loses 
its brilliant colors, and dries away to a shapeless, shrivel- 
ing, sticky mass. 
Some of the most beautiful of the jelly-fishes belong 
to a group called the Siphonophora. These jelly-fishes are 
elongate and tube-like rather than umbrella- or bell-shaped, 
and they are polymorphic—that is, there are several dif- 
ferent forms of individuals belonging to a single kind 
or species. The Siphonophora are all free-swimming, but 
nevertheless form small colonies. In the Mediterranean 
Sea and in other southern ocean waters the surface may be 
covered for great areas by these brilliantly colored jelly-fish 
colonies, each of which looks, as a celebrated German natu- 
ralist has said, like a swimming flower cluster whose parts, 
flowers, stems, and leaves seem to be made of transparent 
crystal, but which possess the life and soul of an animal. 
An abundant species of these Siphonophora (Fig. 25) is com- 
posed of a slender, flexible, floating, central stem several feet 
long, to which are attached thousands of medusa and polyp 
individuals representing several different kinds of forms, 
each kind of individual being specially modified or adapted 
to perform some one duty. The central stem is a greatly 
elongated polyp individual, whose upper end is dilated and 
filled with air to form a float. This individual holds up 
the whole colony. Grouped around this central stem just 
below the float are many bell-shaped bodies which alter- 
nately open and close, and by thus drawing in and expelling 
water from their cavities impel the whole colony through 
the water. These bell-shaped structures are attached me- 
