788 
THE VOYAGE OF H.M.S. CHALLENGER. 
dead corals of other species. The tips of the lobes of the living coral are of a bright 
gamboge-yellow colour, which shades off into a yellowish brown on either side of the 
lobes. Mr. Murray succeeded in getting the polyps of the coral to expand under the 
microscope, and handed them over to me for examination. I found them, as Agassiz 
had discovered long before, to be Hydroids allied to the Medusae, and not Actinozoa 
allied to sea anemones, like the majority of modern stony corals ; I studied the structure 
of the coral minutely. 1 
“ The hard part of the coral or calcareous skeleton is finely porous throughout, being 
excavated by a complex reticulation of fine and tortuous canals which are in the freest 
possible communication with one another. Within this porous mass at its surface are 
excavated cylindrical holes or pores of two sizes. The canal spaces in the skeleton are, 
when the coral is living, filled by a network of living tissue made up of a meshwork of 
branching and communicating tubes, which form a canal system, by means of which a 
free circulation can pass from one part of the coral to another. 
“ Two kinds of polyps inhabit the pores described as existing on the surface of the 
coral. The larger pores are occupied by short stout cylindrical polyps which have each four 
tentacles and a mouth and stomach, and which are hence termed ‘ Gastrozooids/ whilst 
their pores are termed ‘ Gastropores.’ The smaller pores shelter each a very different 
kind of polyp, which has a long and slender sinuous body provided with numerous 
tentacles, and devoid of any mouth or stomach; this latter form of polyp, because its 
function is merely to catch food, is called a ‘Dactylozooid/ and its pore a ‘Dactylopore.’ 
“ The dactylozooids catch food for the colony and deliver it to the gastrozooids, which 
alone are able to swallow and digest it. All the polyps of the colony are in communica- 
tion at their bases with the canal system already described, and by means of these canals 
the nutritive fluids derived by the gastrozooids from the food are distributed to the entire 
colony and nourish it ; there is thus a very complete division of labour in the colony. 
“ In all species of Mill&pora the mouth-bearing polyps are much less numerous than 
the mouthless ones. In some species the gastropores and dactylopores are scattered 
irregularly over the surface of the colonies. In the Tahitian species, however, they are 
for the most part gathered into definite groups or systems, each consisting of a centrally 
placed gastropore surrounded by a ring of five, six, or seven dactylopores, as shown in 
the accompanying figure (see fig. 274), where the circular groups of minute pores are 
seen scattered over the coral surface. 
“Fig. 275 shows, much enlarged, a single system of polyps belonging to one of the 
pore systems, as it appears when the polyps are fully protruded from their pores and 
expanded. Beneath is seen shaded dark, part of the canal meshwork, which maintains 
the general circulation of the colony. From this stands up in the centre, the short 
1 For further details see H. N. Moseley, On the Structure of a Species of Millepora occurring at Tahiti, Society 
Islands, Phil. Trans., vol. clxvii. pp. 117-135, 1877. 
