The gastrozoids usually bear short capitate tentacles, four, six or twelve in number; but in Astylus (fig. 63) they have no tentacles. The dactylozoids have no mouth; in Milleporidae they have short capitate tentacles, but lack tentacles in Stylasteridae.
The gonosome consists of free medusae in Milleporidae, which are budded from the apex of a dactylozoid in Millepora murrayi, but in other species from the coenosarcal canals. The medusae are produced by direct budding, without an entocodon in the bud. They are liberated in a mature condition, and probably live but a short time, merely sufficient to spread the species. The manubrium bearing the gonads is mouthless, and the umbrella is without tentacles, sense-organs, velum or radial canals. In the Stylasteridae sessile gonophores are formed, always by budding from the coenosarc. In Distichopora the gonophores have radial canals, but in other genera they are sporosacs with no trace of medusoid structure.
| Fig. 63.—Portion of the corallum of Astylus subviridis (one of the Stylasteridae), showing cyclosystems placed at intervals on the branches, each with a central gastropore and zone of slit-like dactylopores. (After Moseley.) |
Classification.—Two families are known:—
1. Milleporidae.—Coenosteum massive, irregular in form; pores scattered irregularly or in cyclosystems, without styles, with transverse tabulae; free medusae. A single genus, Millepora (figs. 60, 61).
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.