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 this Medusa, like many others of the order, undergoes considerable changes in form before it reaches the sexually mature stage.
Phialidium temporarium is one of the commonest Medusae of our coast, and sometimes occurs in shoals. It seems probable that it is the Medusa of Clytia johnstoni.[[319]] By some authors the jelly-fish known as Epenthesis is also believed to be the Medusa of a Clytia.
Fam. Dendrograptidae.—This family includes a number of fossils which have certain distinct affinities with the Calyptoblastea. In Dictyonema, common in the Ordovician rocks of Norway, but also found in the Palaeozoic rocks of North America and elsewhere, the fossil forms fan-shaped colonies of delicate filaments, united by many transverse commissures, and in well-preserved specimens the terminal branches bear well-marked uniserial hydrothecae. In some species thecae of a different character, which have been interpreted to be gonothecae and nematophores respectively, are found.
Other genera are Dendrograptus, Thamnograptus, and several others from Silurian strata.
Order V. Graptolitoidea.
A large number of fossils, usually called Graptolites, occurring in Palaeozoic strata, are generally regarded as the skeletal remains of an ancient group of Hydrozoa.
In the simpler forms the fossil consists of a delicate straight rod bearing on one side a series of small cups. It is suggested that the cups contained hydroid zooids, and should therefore be regarded as the equivalent of the hydrothecae, and that the axis represents the axis of the colony or of a branch of the Calyptoblastea. In some of the forms with two rows of cups on the axis (Diplograptus), however, it has been shown that the cups are absent from a considerable portion of one end of the axis, and that the axes of several radially arranged individuals are fused together and united to a central circular plate. Moreover, there is found in many specimens a series of vesicles, a little larger in size than the cups, attached to the plate and arranged in a circle at the base of the axes. These vesicles are called the gonothecae.
The discovery of the central plate and of the so-called gonothecae suggests that the usual comparison of a Graptolite with a Sertularian Hydroid is erroneous, and that the colony or individual, when alive, was a more or less radially symmetrical floating form, like a Medusa, of which only the distal appendages (possibly tentacles) are commonly preserved as fossils.
The evidence that the Graptolites were Hydrozoa is in reality very slight, but the proof of their relationship to any other phylum of the animal kingdom does not exist.[[320]] It is therefore convenient to consider them in this place, and to regard them, provisionally, as related to the Calyptoblastea.