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.
Fam. Hydroceratinidae.—The colony consists of a mass of entwined hydrorhiza, with a skeleton in the form of anastomosing chitinous tubes. Hydrothecae scattered, tubular, and sessile. Nematophores present. Gonophores probably adelocodonic.
This family was constituted for a remarkable hydroid, Clathrozoon wilsoni, described by W. B. Spencer from Victoria.[[317]] The zooids are sessile, and spring from more than one of the numerous anastomosing tubes of the stem and branches. The whole of the surface is studded with an enormous number of small and very simple dactylozooids, protected by tubular nematophores. Only a few specimens have hitherto been obtained, the largest being 10 inches in height by 4 inches in width. In general appearance it has some resemblance to a dark coloured fan-shaped Gorgonia.
Fam. Campanulariidae.—The hydrothecae in this family are pedunculate, and the gonophores adelocodonic.
In the cosmopolitan genus Campanularia the stem is monosiphonic, and the hydrothecae bell-shaped. Several species of this genus are very common in the rock pools of our coast between tide marks. Halecium is characterised by the rudimentary character of its hydrothecae, which are incapable of receiving the zooids even in their maximum condition of retraction. The genus Lafoea is remarkable for the development of a large number of tightly packed gonothecae on the hydrorhiza, each of which contains a blastostyle, bearing a single gonophore and, in the female, a single ovum. This group of gonothecae was regarded as a distinct genus of Hydroids, and was named Coppinia.[[318]] Lafoea dumosa with gonothecae of the type described as Coppinia arcta occurs on the British coast.
Perisiphonia is an interesting genus from deep water off the Azores, Australia, and New Zealand, with a stem composed of many distinct tubes.
The genus Zygophylax, from 500 fathoms off the Cape Verde, is of considerable interest in having a nematophore on each side of the hydrotheca. According to Quelch it should be placed in a distinct family.
Ophiodes has long and very active defensive zooids, protected by nematophores. It is found in the Laminarian zone on the English coast.
Fam. Eucopidae.—The hydrosome stage of this family is very similar to that of the Campanulariidae, but the gonophores are free-swimming Medusae of the Leptomedusan type.