100 THE VOYAGE OF H.M.S. CHALLENGER. 
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 ccenosteum. 
In Sporadopora, the most ancestral Stylasterid at present known, the styles of the 
dactylopores have disappeared, and they only reappear apparently by reversion in 
Allopora and Stylaster. Rudimentary tabule 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 specialised members 
of the family Astylus and Cryptohelia.  Distichopora appears to have been derived 
directly from some form allied to Errina. 
Two separate modifications of the nariform projections of Evrina 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 Saville Kent’s figures,’ or to the diagrams given on Plate I. 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 probable that 
this condition has been brought about by the continual bending inwards of the dactylo- 
pores 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 the pores are found 
to be in all the systems still entirely independent (fig. 12), and this condition is 
maintained at greater depths in all Stylasters. It is to be noted that in becoming 
so remarkably modified into elongate slit-like cavities, the dactylopores of Stylasteride 
with cyclo-systems follow an ancestral tendency to modification, for these elongate pores 
are, taken separately, closely similar in form to the nariform dactylopores of Evrina 
and its alles: they only have the lips of all the projections directed radially out- 
wards. 
' Proc. Zool. Soc., 1871, pl. xxv. fig. 2a. 
