In this singular class of fresh-water and oceanic Hydrozoa, the internal cilia, aided by the contraction of the walls of the body, are the sole means provided by nature for the circulation of the fluids. SECTION IV. ANTHOZOA ZOOPHYTES. THE life-history of the oceanic Hydrozoa, which may be regarded as one of the triumphs of microscopic science, would have been incomplete had it been separated from that of the Pulmograde Acalephæ and the Physograde groups: but the most important part of that numerous race of animals are the Anthozoa zoophytes, which include the builders of the coral reefs and atolls of the Indian and Chinese seas. The coral polypes, though feeble and inconspicuous individually, when united in large communities acquire a power which enables them to build the most stupendous structures in the midst of a tempestuous ocean. The Anthozoa zoophytes, or living flowers, form two extensive groups—the Asteroids, or Alcyonian zoophytes, whose polypes have six or eight hollow prehensile tentacles radiating round their mouth like a star or the petals of a single blossom, and the Actinian or Helianthoid zoophytes, which have ten, twelve, or more hollow tentacles encircling their mouth in several rows, like the blossom of a double sun-flower. _Alcyon Zoophytes._ The Alcyon zoophytes comprise the Alcyons, Gorgons, and Pennatulidæ, or sea-pens. The polypes are of the same form in all, and are united by a fleshy or horny substance into large communities, so connected and mutually dependent as to constitute one compound animal. Figure 123 represents a highly magnified group of Alcyonian polypes in different stages of expansion. The body of the polype is soft, contractile, and composed of thin, delicate transparent tissues. It has the form of a cone resting upon its base, which is generally of a firm material. Its upper extremity presents a central orifice, which serves both for a mouth and vent, and is encompassed by six or eight broad, short, hollow tentacles, enlarged towards their base so as to meet, and their edges are seen with a lens only to be fringed with minute hollow tubes or pinnæ closed at their free end. [Illustration: Fig. 123. Alcyonian polypes highly magnified.] [Illustration: Fig. 124. Polype of Alcyonidium elegans.]