_Nectocalyces._—The nectocalyces are in the form of the umbrella of a medusa attached to the stolon of the colony by the aboral pole. They are provided with a velum and, usually, four radial canals and a circular canal. There is no manubrium, and the marginal tentacles and sense-organs are rudimentary or absent. There may be one or more nectocalyces in each colony, {299}and their function is, by rhythmic contractions, to propel the colony through the water (Fig. 142, N). _Gastrozooids._—These are tubular or saccular zooids provided with a mouth and attached by their aboral extremity to the stolon (Fig. 142, G). In some cases the aboral region of the zooid is differentiated as a stomach. It is dilated and bears the digestive cells, the oral extremity or hypostome being narrower and more transparent. In some cases the mouth is a simple round aperture at the extremity of the hypostome, but in others it is dilated to form a trumpet-like lip. _Dactylozooids_.—In _Velella_ and _Porpita_ the dactylozooids are similar in general characters to the tentacles of many Medusae. They are arranged as a frill round the margin of the colony, and each consists of a simple tube of ectoderm and endoderm terminating in a knobbed extremity richly provided with nematocysts. In many other Siphonophora, however, the dactylozooids are very long and elaborate filaments, which extend for a great distance from the colony into the sea. They reach their most elaborate condition in the Calycophorae. [Illustration: FIG. 141.—A small Crustacean (_Rhinocalanus_) caught by a terminal filament (_f.t_) of a battery of _Stephanophyes_. _b_, The proximal end of the battery with the most powerful nematocysts; _e_, elastic band; _S_, stalk supporting the battery on the dactylozooid. (After Chun.)]