64 THE VOYAGE OF HM.S, CHALLENGER. 
They appear to be homologous with the radial canals already described as occurring in 
a similar connection in Sporadopora dichotoma (Pl. X. fig. 3). In transverse sections of 
a zooid system, these radial offsets have much the appearance of the mesenteries of an 
Anthozoan coral cut across, and in some sections they show a certain amount of regularity 
in disposition at the particular level selected for the cut (Pl. XI. fig. 12). 
These radial structures are here termed offsets, and not canals, because, although in 
some instances they appear to be similar in construction to the ordinary ccenosarcal 
canals, and usually show similar structure to these at their outer extremities, they 
usually consist towards their middles and inner extremities of simple bands of trans- 
parent fibrous tissue. The exact structure could not be determined, but it seems 
probable that these radial bands represent radially disposed offsets of the canal mesh- 
work, which have become developed into fibrous organs with an elastic or muscular 
function, which is brought to bear on the zooid sac. In the case of the radial canals 
of Sporadopora dichotoma, distinct muscular elements were observed as forming part 
of their structure. 
Dactylozooids.—The dactylozooids and their sacs in Allopora profunda are so 
closely similar in form and structure to those of Stylaster densicaulis, that they need no 
further description. 
Gastrozooids.—The sacs of the gastrozooids in the present species differ from those 
in Stylaster densicaulis in being of smaller diameter in proportion to the dimensions of 
the pore cavities, and in being held in place by means of the radial offsets already 
described. It is possible that the wall of the sac of the gastrozooid lies nearer to the 
wall of the gastropore in the recent condition of the coral than is represented to be 
the case in Plate XXXIX. GZ, and in Plate XI. fig. 12; but all the spirit specimens 
examined yielded a similar result when decalcified. The sac of the gastrozooid is, as 
usual, a reflection of the surface layer of the ectoderm. In the contracted condition it 
forms a long tubular cavity, somewhat narrowed in the region just above the gastrozooid 
and at the mouth, and dilated towards the centre. At its mouth, the marginal fold of 
the sac rises in the form of a flattened dome somewhat above the level at which the 
openings of the sacs of the dactylozooids commence. 
The gastrozooids are very deeply seated at the bottoms of their sacs. They are 
dome-like in form, with expanded bases. A whorl of tentacles, set on at some distance 
from the summit of the dome, marks the commencement of the large rounded hypostome. 
The tentacles are twelve in number, and are set on in a single whorl. They are 
elongate-ovoid in form. From the base of the zooid a series of radially disposed large 
canals pass outwards to be distributed as in Stylaster densicaulis, a certain number of 
their branches forming a tortuous meshwork, offsets from which pass to join those of 
the gastrozooid of the adjacent system. 
The structure of the zooid eyclo-systems is clearly displayed in Plate XI. fig. 12, which 
