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 {280}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. One of the best-known genera is _Obelia_, of which several species are among the commonest Hydroids of the British coast. _Clytia johnstoni_ is also a very common Hydroid, growing on red algae or leaves of the weed _Zostera_. It consists of a number of upright, simple, or slightly branched stems springing from a creeping hydrorhiza. When liberated the Medusae are globular in form, with four radial canals and four marginal tentacles, but {281}this Medusa, like many others of the order, undergoes considerable changes in form before it reaches the sexually mature stage.