MR. H. N. MOSELEY ON THE STRUCTURE OF THE STYLASTERID2E. 
483 
their gonophores have been investigated, it will follow that the two families have had a 
common ancestor, and that Hydroids have developed a calcareous support only once 
in their history and not in two separate instances. This common ancestor may be 
presumed to have had a hydrosoma composed, as throughout the sub-order Hydro- 
corallinse ; with its pores sporadic, with tabulse and without styles, and with two kinds 
of zooids, with knob-bearing tentacles ; with a tendency also of the dactylozooids to 
form ring-like groups around a gastrozooid. 
From this form Archistylaster was developed with a branching corallum; with a 
strong tendency to assume a flabellate form, and to develop its pores only on one face 
of the flabellum, or at the sides only of the branches ; with its pores sporadic and 
tabulate, and styles in both forms of them. The dactylozooids of Archistylaster were 
devoid of the knobbed tentacles, these were, however, retained by the gastrozooid. 
The gonangia were included in hollows in the corallum. 
In Sporadopora, the most ancestral Slylasterid at present known, the styles of the 
dactylopores have disappeared, and they only reappear apparently by reversion in 
Allopora and Stylcister. Rudimentary tabula are present in Sporadopora and Plio- 
bothrus, but disappear in succeeding genera. In Pliobothrus the margins of the dac 
tylopore mouths are raised up and prolonged into small tubuli, and the genus would 
thus lead to Errina , where the tubuli become nariform, were it not that in Pliobothrus 
the style of the gastrozooid is lost, and that the gastrozooid is devoid of tentacles and 
flask-shaped : a condition occurring again only in the most highly specialized members 
of the family Astylus and Cryptohelia. 
Two separate modifications of the nariform projections of Errina are presented by 
Porella and Spinipora, in both which genera further complication ensues by the 
differentiation of two kinds of dactylozooids. 
The process of the formation of cyclo-systems is seen in all stages in different parts 
of the surface of the single species Allopora subviolacea, as will be seen by reference 
to Savile Kent’s figures,* or to the diagrams given on Plate 44 of the present 
Memoir, figs. 10, 11, and 12. In this coral five or six dactylopores are grouped 
in a circle around a single centrally placed gastropore. In some groups all the pores 
are simply circular (fig. 10). In others, shallow grooves, often only just indicated, 
lead radially from the dactylopores towards the gastropore. In others, these grooves 
are well marked and deep, and a complete cyclo-system is formed. It appears pro- 
bable that this condition has been brought about by the continual bending inwards of 
the dactylopores to convey food to the gastropore. The grooves have been the result 
of the attempts of these zooids to reach the gastrozooid when further and further 
retracted. Thus, in most Alloporas and all Stylasters, all the pores have come to 
form regular cyclo- systems, in which the mouths of the dactylopores are drawn out 
into elongate chambers, and their tubular prolongations reduced to mere rudiments 
in many cases. At a very short distance below the surface in Allopora subviolacea 
* Proc. Zool. Soc., 1871, Plate 25, fig. 2 a. 
