=Classification.=--The branch Cœlenterata is divided into four classes: (1) the Hydrozoa, including the fresh-water polyps, numerous marine polyps, many small jellyfishes and a few corals; (2) the Scyphozoa, including most of the large jellyfishes; (3) the Actinozoa, including the sea-anemones and most of the stony corals; (4) the Ctenophora, including certain peculiar jellyfishes. [Illustration: FIG. 13.--The Portuguese Man-of-War (_Physalia_ sp.). (From specimen from Atlantic Coast.)] =The polyps, colonial jellyfishes, etc. (Hydrozoa).=--To the class Hydrozoa belongs the _Hydra_ already studied. There are a few other fresh-water polyps and they all belong to this class. The most interesting members of the class are the "colonial jellyfishes," constituting the order Siphonophora. These colonial jellyfishes are floating or swimming colonies of polypoid and medusoid individuals in which there is a marked division of labor among the individuals, accompanied by marked differences in structural character. The individuals are accordingly polymorphic, that is, appear in various forms, although all belong to the same species. Because these various individuals forming a colony have given up very largely their individuality, combining together and acting together like the organs of a complex animal, they are usually not called individuals, nor on the other hand organs, but _zooids_, or animal-like structures. The beautiful "Portuguese man-of-war" (fig. 13) is one of these colonial jellyfishes. It appears as a delicate bladder-like float, brilliant blue or orange in color, usually about six inches long, and bearing on its upper surface which projects above the water a raised parti-colored crest, and on its under surface a tangle of various appendages, thread-like with grape-like clusters of little bell- or pear-shaped bodies. Each of these parts is a peculiarly modified polyp- or medusa-zooid produced by budding from an original central zooid. The Portuguese man-of-war is very common in tropical oceans, and sometimes vast numbers swimming together make the surface of the ocean look like a splendid flower-garden. [Illustration: FIG. 14.--A colonial jellyfish (Siphonophora). (After Haeckel.)]