The members of this order are closely allied to the Sertularida; so closely, indeed, that they are very often united together into a single group. The chief difference consists in the fact that the hydrothecae of the Campanularida, with their contained polypites, are supported upon conspicuous stalks, thus being terminal in position (figs. 46, b, and 50); whilst in the Sertularida they are sessile or subsessile, and are placed laterally upon the branchlets. The gonophores also in the Campanularida are usually detached as free-swimming medusoids, whereas they remain permanently attached in the Sertularians. Each medusoid consists of a little transparent glassy bell, from the under surface of which there is suspended a modified polypite, in the form of a "manubrium" (fig. 51). The whole organism swims gaily through the water, propelled by the contractions of the bell or disc (gonocalyx); and no one would now suspect that it was in any way related to the fixed plant like zoophyte from which it was originally budded off. The central polypite is furnished with a mouth at its distal end, and the mouth opens into a digestive sac. From the proximal end of this stomach proceed four radiating canals which extend to the circumference of the disc, where they all open into a single circular vessel surrounding the mouth of the bell. From the margins of the disc hang also a number of delicate extensile filaments or tentacles; and the circumference is still further adorned with a series of brightly-coloured spots, which are probably organs of sense. The mouth of the bell is partially closed by a delicate transparent membrane or shelf, the so-called "veil." Thus constituted, these beautiful little beings lead an independent and locomotive existence for a longer or shorter period. Ultimately, the essential elements of reproduction are developed in special organs, situated in the course of the radiating canals of the disc. The resulting embryos are ciliated and free-swimming, but ultimately fix themselves, and develop into the plant-like colony from which fresh medusoids may be budded off. The ova in the medusiform gonophores are usually developed in the course of the gonocalycine canals, and not between the ectoderm and endoderm of the manubrium, as is the case in the Corynida. Examples of the order are Campanularia, Laomedea, etc. The distinctions between the Sertularida and Campamdarida are certainly insufficient to justify their being placed in separate orders. If united together, it would probably be best to adopt the name of Calyp-toblastica (Allman) or Thecaphora (Hincks) for the order, and to employ the names Sertularida and Campanularida for the sub-orders.
Fig. 51. - Free medusiform gono-phore of Clytia fohnstoni (after Hincks). a Central polypite or manubrium; b b Radiating gastro-vascular canals; c Circular canal; M Marginal bodies; t Tentacles.