ACTINOZOA. 97 
reef on the N.E. coast of Australia, the structure of which is on a gigan- 
tic scale. This reef runs, with a few trifling interruptions, for a distance 
of more than a thousand miles, with an average breadth of thirty miles, 
and an area of thirty-three thousand square miles. Its average distance 
from the shore is between twenty and thirty miles, the depth of the 
inner channel is from ten to sixty fathoms, and the sea outside is ¢ pro- 
foundly deep” (in some places over eighteen hundred feet). 
3. Atolls (fig. 36, 3).—These are oval or circular reefs of coral enclos- 
ing a central expanse of water or lagoon. They seldom form complete 
rings, the reef being usually breached by one or more openings. They 
agree in all particulars with those barrier-reefs which surround islands, 
except that there is no central island in the lagoon which they enclose. 
The last group of the Zoan/haria comprises composite organ- 
isms in which the coenosarc is supported upon a central axis or 
sclerobasic skeleton. These Zoantharia sclerobasica require no 
notice, except simply to remark that they are distinguished 
from other sclerobasic corals (such as the Gorgonide) by the 
fact that each polype possesses tentacles which are a multiple 
of s¢zxin number. ~ 
ORDER II. ALcyonariA.—The second great order of living 
Actinozoa is distinguished by the fact that the polypes are 
furnished with /rzmged tentacles, and that these, as well as the 
mesenteries and somatic chambers, are always some multiple 
of four. With one doubtful exception, all the Adcyonaria are 
composite, their polypes being connected together by a cceno- 
sarc. The body-cavities of the polypes are connected with a 
system of canals which are excavated in the ccenosarc, and 
communicate freely with one another, so that a free circulation 
of nutrient fluids is thus kept up. The structure of the polypes 
of the Alcyonaria is, in all essential anatomical features, the 
same as in the sea-anemones, the number of the mesenteries 
and tentacles being the chief distinction. 
Of the various different organisms included under this order, 
one of the best known is the “ Dead-men’s-fingers,” or A lcyo- 
mzum, which occurs commonly in our seas, It forms spongy- 
looking masses of a yellow or orange colour, attached to shells 
and other marine objects. The whole mass is covered with 
little star-shaped apertures, through which the delicate polypes 
can be protruded and retracted at will. Another well-known 
member of this order—the type of another family—is the “ Sea: 
G 
