42 THE VOYAGE OF H.M.S. CHALLENGER. 
forms a sac open above. ‘The open mouth of the sac is crammed with nematocysts of the 
larger kind, closely packed side by side, with their pointed ends directed to the surface. 
The cells are so closely packed that, in a section of the superficial layer taken parallel to 
the surface through the nematophores, no interstices can be seen between them (Pl. 
X. fig. 3, N). The lower part of the cavity of the nematophore is filled with nuclei 
and parent cells of the nematocysts. The nematophores, as viewed from the surface of 
the superficial layer, are seen to have an irregular outline, showing a tendency to be 
somewhat oblong, with curved boundaries. 
No triple-spined nematocysts, such as those occurring in Millepora and in most 
other Hydroids, were detected as existing in any of the Stylasteride. The two 
kinds described as occurring in Sporadopora dichotoma appear to be present in all 
members of the family, with very slight variations in form indeed. 
Dactylozooids—The dactylozooids are in all the Stylasteride invariably destitute of 
tentacles, being reduced to the condition of simple tentacles themselves, and evidently 
performing a tentacular function. 
Zooids.—The zooids in Sporadopora dichotoma are of two kinds, dactylozooids and 
gastrozooids : the former occupying the smaller, and the latter the larger, style-bearing 
pores, already described as characteristic of the corallum. 
They are closely similar in form and structure in all the genera of Stylasteride 
hitherto examined, and differ only in dimensions. They are simple, elongate, 
conical bodies, just like the ordinary tentacles of Hexactinians in form, and are 
devoid of mouth or any opening to the exterior. They are attached to, and, when 
unprotruded, retracted within membranous sacs or sheaths which rest within the 
corresponding pores of the corallum. In Sporadopora, the sacs of the zooids, the 
walls of which are shown in longitudinal section in Plate III. F F, are composed of a 
transparent membrane, derived from the ectoderm, and continuous with its surface 
layer. The membrane has numerous fine nuclei dispersed in its substance, and is 
strong and tough. It is lined on its inner surface next the cavity of the sac by a 
layer of small transparent cells, which are shown in the figure cited above. 
On their outer surfaces the walls of the zooid sacs are abutted on by the peculiar 
radial offsets of the ccenosarcal meshwork already described. These offsets appear to 
lose their tubular character as they near the walls of the sacs, and, as far as was seen, 
no openings occur in the sac walls communicating by means of these radial canals with 
the ccenosarcal circulation, although such an arrangement was supposed to exist when 
the first hasty examination of specimens was made, 
The sacs are attached to the bases of the zooids, being continuous in those regions 
with the ectodermal covering of the zooids. They closely invest the retracted zooids, 
and are thus cylindrical in form in their deeper parts; whilst above the level of the 
retracted zooids they contract gradually in diameter, to terminate at the surface of the 
