2. _Stylasteridae._--Coenosteum arborescent, sometimes fanlike, with pores only on one face, or on the lateral margins of the branches; gastropores with tabulae only in two genera, but with (except in _Astylus_) a _style_, i.e. a conical, thorn-like projection from the base of the pore, sometimes found also in dactylopores; sessile gonophores. _Sporadopora_ has the pores scattered irregularly. _Distichopora_ has the pores arranged in rows. _Stylaster_ has cyclosystems. In _Allopora_ the cyclostems resemble the calyces of Anthozoan corals. In _Cryptohelia_ the cyclosystem is covered by a cap or operculum. In _Astylus_ (fig. 63) styles are absent. _Affinities of the Hydrocorallinae._--There can be no doubt that the forms comprised in this order bear a close relationship to the Hydroidea, especially the sub-order Gymnoblastea, with which they should perhaps be classed in a natural classification. A hydrocoralline may be regarded as a form of hydroid colony in which the coenosarc forms a felt-work ramifying in all planes, and in which the chitinous perisarc is replaced by a massive calcareous skeleton. So far as the trophosome is concerned, the step from an encrusting hydroid such as _Hydractinia_ to the hydrocoralline _Millepora_ is not great. Hickson considers that the families _Milleporidae_ and _Stylasteridae_ should stand quite apart from one another and should not be united in one order. The nearest approach to the _Stylasteridae_ is perhaps to be found in _Ceratella_, with its arborescent trophosome formed of anastomosing coenosarcal tubes supported by a thick perisarc and covered by a common ectoderm. _Ceratella_ stands in much the same relation to the _Stylasteridae_ that _Hydractinia_ does to the _Milleporidae_, in both cases the chitinous perisarc being replaced by the solid coenosteum to which the hydrocorallines owe the second half of their name. ORDER IV. Graptolitoidea (Rhabdophora, Allman).--This order has been constituted for a peculiar group of palaeozoic fossils, which have been interpreted as the remains of the skeletons of Hydrozoa of an extinct type. A typical graptolite consists of an axis bearing a series of tooth-like projections, like a saw. Each such projection is regarded as representing a cup or hydrotheca, similar to those borne by a calyptoblastic hydroid, such as _Sertularia_. The supposed hydrothecae may be present on one side of the axis only (monoprionid) or on both sides (diprionid); the first case may be conjectured to be the result of uniserial (helicoid) budding, the second to be produced by biserial (scorpioid) budding. In one division (_Retiolitidae_) the axis is reticulate. In addition to the stems bearing cups, there are found vesicles associated with them, which have been interpreted as gonothecae or as floats, that is to say, air-bladders, acting as hydrostatic organs for a floating polyp-colony.