ORDER II. TRACHYLINA The Trachylina are an assemblage of Hydrozoa which differ sufficiently from any of the Hydroida or Siphonophora to warrant their inclusion in a separate group.
Among the Trachylina the medusa is the dominant form, and many reach a considerable size (e.g., io x 3cm.). In accordance with this fact the Trachylina are mostly oceanic forms, pelagic throughout life; whilst the Hydroida are tethered, so far as their polyp-generation is concerned, to the bottom or to sea-weed, and include many characteristic shore-forms. The cleavage of the fertilized egg of the Trachylina typically produces a planula (see COELENTERATA), which develops into a more or less distinctly polyp-like larva; the latter is jransformed directly into a medusa (fig. 8) . In certain forms (Cuninidae) the polyp-larva is para sitic within medusae of its own or other kinds. In such cases it reproduces by budding, and both the parent and daughter polyps become transformed into medusae ; or, the larva may form a stolon from which medusae are budded.
Beyond the facts thus outlined the Trachylina contribute little of general interest to the study of the Coelenterata, although their structure and life-histories are in themselves extremely in teresting. For this reason they are dealt with very briefly here, and the only part of their structure calling for further mention is that of the sense organs. Ocelli (eye-spots) are rare amongst them, but all possess organs containing statoliths. These are of a different grade from those of the Hydroida, in that they exhibit the structure, not of ectodermal pits or sacs containing statoliths, but of small tentacles containing an endodermal core in which lie one or more statoliths, and covered by ectoderm. These modified tentacles are known as tentaculocysts (fig. 9), and they may, like tentacles, project freely at the surface, or may themselves become embedded in a pit or sac formed by the surrounding tissues.