52 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 ampullae. 



The mass of the ccenosteum is composed, as in other Stylasteridas, of hard calcareous 

 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 

 (PI. 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 JErrina lahiata (PI. IV.). 



Cosnosarc. — The ccenosarcal meshwork in Errina lahiata is more widely open in its 

 structure than in Sporadopora dichotoma (PI. 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 

 ccenosarc retain their vitality even to the very axes of the branches. The general 

 arrangement of the ccenosarcal 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 adjoining the sacs 

 is to be observed, representing the more regular radial disposition described as existing 

 in Sporadopora dichotoma. 



The histological structure of the ccenosarcal 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 sjsecimens 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 (PI. XL 

 fig. 10). 



