FAM. SERTULARIIDAE.—In this family the hydrothecae are sessile, and arranged bilaterally on the stem and branches. The general form of the colony is pinnate, the branches being usually on opposite sides of the main stem. The gonophores are adelocodonic. _Sertularia_ forms more or less arborescent colonies, springing from a creeping stolon attached to stones and shells. There are many species, several of which are very common upon the British coast. Many specimens are torn from their attachments by storms or by the trawls of fishermen and cast up on the sand or beach with other zoophytes. The popular name for one of the commonest species (_S. abietina_) is the "sea-fir." The genus has a wide geographical and bathymetrical range. Another common British species frequently thrown up by the tide in great quantities is _Hydrallmania falcata_. It has slender spirally-twisted stems and branches, and the hydrothecae are arranged unilaterally. The genus _Grammaria_, sometimes placed in a separate family, is distinguished from _Sertularia_ by several characters. The stem and branches are composed of a number of tubes which are considerably compressed. The genus is confined to the southern seas. {279}FAM. PLUMULARIIDAE.—The hydrothecae are sessile, and arranged in a single row on the stem and branches. Nematophores are always present. Gonophores adelocodonic. This family is the largest and most widely distributed of all the families of the Hydrozoa. Nutting calculates that it contains more than one-fourth of all the Hydroids of the world. Over 300 species have been described, and more than half of these are found in the West Indian and Australian regions. Representatives of the family occur in abundance in depths down to 300 fathoms, and not unfrequently to 500 fathoms. Only a few species have occasionally been found in depths of over 1000 fathoms. The presence of nematophores may be taken as the most characteristic feature of the family, but similar structures are also found in some species belonging to other families (p. 277). The family is divided into two groups of genera, the ELEUTHEROPLEA and the STATOPLEA. In the former the nematophores are mounted on a slender pedicel, which admits of more or less movement, and in the latter the nematophores are sessile. The genera _Plumularia_ and _Antennularia_ belong to the Eleutheroplea. The former is a very large genus, with several common British species, distinguished by the terminal branches being pinnately disposed, and the latter, represented by _A. antennina_ and _A. ramosa_ on the British coast, is distinguished by the terminal branches being arranged in verticils. The two most important genera of the Statoplea are _Aglaophenia_ and _Cladocarpus_. The former is represented by a few species in European waters, the latter is only found in American waters.