2 THE VOYAGE OF H.M.S. CHALLENGER. 
breaks away entirely, allowing the escape of the imprisoned planula. The empty hollows 
remaining after this process is complete are abundantly present on the surfaces of the 
branches, and are often to be seen remaining on the older regions of the main stems, 
although in these older regions there is a tendency to obliteration, by interstitial 
calcareous deposit, of all pores and ampulle. 
The mass of the ccenosteum is composed, as in other Stylasteride, of hard caleareous 
tissue permeated in all directions by meshworks of canals. The canals generally are, in 
the present genus, larger in proportion to the size of the zooids than in most other forms 
(Pl. IV.), and the meshworks formed by them are comparatively widely open. The 
main canals have a general tendency to traverse the axes of the stems and branches, 
spreading out at an inclination corresponding with that of the pure cavities towards the 
surfaces. This arrangement necessarily results from the mode of growth. In the older 
regions of the stem the ccenosteum becomes more compact and stony by obliteration of 
many of the canals, but the main canals appear never to become entirely obliterated 
even very low down towards the bases of the stems. 
Soft structures of Errina labiata (PI. IV.). 
Cenosarc.—The ceenosarcal meshwork in Errina labiata is more widely open in its 
structure than in Sporadopora dichotoma (Pl. IV.). Hence the mass of soft structures 
separated from the ccenosteum by decalcification is comparatively soft and less able to 
maintain the original form of the corallum. In the present species, however, in all the 
actively living branches it is not, as in Sporadopora dichotoma, a mere surface layer of 
the coral which is living supported by dead ccenosteum below, but the deeper canals of the 
coenosare retain thei vitality even to the very axes of the branches. The general 
arrangement of the coenosarcal canals is seen in Plate IV. Closer meshworks compose the 
mass near the surface, and in deeper regions the canals are larger and form wider and 
longer meshes, and constitute an axial system of main canals by which the various 
distant zooids are brought into relation with one another. Around the sacs of the 
gastrozooids an irregular radial arrangement of the canals immediately adjoiming the sacs 
is to be observed, representing the more reeular radial disposition described as existing 
in Sporadopora dichotoma. 
The histological structure of the ccenosareal canals is closely similar to that occurring 
in those of Sporadopora. The endodermal pigmented cells are of a light brick-red colour, 
and hence the entire coral in the recent state is thus coloured. The pigment is, however, 
soluble in alcohol, and thus quickly extracted in specimens preserved in that fluid, but it 
is insoluble in glycerine. A continuous superficial layer is present on the surface of the 
coral, as shown in Plate IV., and it is composed of polygonal nucleated cells (Pl. XI. 
fig. 10). 
