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-spine d nematocysts, such as those occurring in Millepora and in most 

 other Hydroids, were detected as existing in any of the Stylasteridse. The two 

 kinds described as occurring in Sjwadopora 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 Stylasteridee 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 Stylasteridge 

 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. The)- 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 



