The second order of the Zoophyta, Asteroida, are those which have a star-like shape; they are congregated into compound structures, forming what are called sea-fans (Gorgonia, fig. 6). These Gorgonia are of all colours, and very beautiful; the animals or "polypes" are gelatinous, and have a gelatinous mass or stalk with branches, strengthened and supported by a horny centre. This gelatinous mass forms the animal itself, as well as a sort of common integument connecting the whole family of many thousands. The Gorgonia or sea-fans usually preserved, are dry and hard, as the gelatinous covering has so little substance that, when dried, it forms a rough powdery covering only to the horny support. This powdery surface, if washed off with a little water and placed under the microscope, is found to contain and be made up of spiculæ or crystals of carbonate of lime, of various colours and beautiful forms (fig. 7); these, when the animal is alive, help to support its gelatinous frame; the Tubiporæ also belong to this class. Fig. 8 represents the Organ-coral (Tubipora musica); it is of a bright red colour, and, when the polypes are living, is a very beautiful object. [Illustration: FIG. 7.--SPICULES OF GORGONIA, MAGNIFIED.] The third order of Zoophytes is the Helianthoida, which include the "Actinia." These Zoophytes are entirely gelatinous, and have beautiful variations of colour. They throw out tentacles like the petals of a flower, and form very beautiful objects; they may be seen in the aquatic vivaria of the Zoological Gardens. To this order of Zoophytes belong the "corals" or Zoanthoid polypes, which produce the coral reefs, extending in some cases hundreds of miles. Dr. Baird gives the following account of them:--