CORALS. 
5 11 
skeletons, the method of secreting which will be described when we come to 
the horny skeletons of the next group. The polyps have only one, instead of 
several rows of tentacles, and in most of them the tentacles are six in 
number. They form compound stocks, looking like delicate shrubs, with long 
branches, from which the polyps project, these branches being supported by a 
flexible horny axis. In the Fiji Islands a stock three feet high, with a stem half 
an inch in thickness, has been found. The general form of the whole stock, the 
brown colouring, and the small, thick tentacles of the little polyps are not 
attractive. 
The Eight-Rayed Polyps, —Order Octactinia. 
Although this second order of the corals contains a variety of forms, the 
appearance of the individual animals is more or less uniform, the number of 
tentacles being always eight. The tentacles are not hollow, but are usually some¬ 
what flattened and notched round the edges like delicate leaves. These corals 
form stocks which are sometimes knobbed or lobed, and sometimes resemble a 
hand or tree with simple branches. The individuals of the stock are usually 
small and rise like minute white blossoms above the soft fleshy surface of the 
stock, which has a peculiar reddish yellow glistening appearance. The stock 
attaches itself by means of a stem, or else rests loosely in the sand, generally 
at a moderate depth. These corals secrete carbonate of lime, but in no case in 
the saine way as do the true corals or the hydrocorals. In both these latter 
the solid skeleton is formed by the outer skin, while in the present group 
the secretion takes the form of minute calcareous particles of definite shape 
scattered about between the outer skin and the lining of the body-cavity. These 
spicules never fuse together to form solid continuous masses, but may, neverthe¬ 
less, be present in sufficient quantities to give the lower part of the body a certain 
degree of rigidity. When fresh, the stocks show some elasticity and turgescence. 
When removed from the water the whole stock contracts strongly, but swells out 
again if placed in an aquarium, where it may live for weeks or months, although 
the great swelling of the lower part shows that its condition is abnormal. A 
tendency to form a stem is also common. The illustration on p. 512 shows an 
alcyonarian, as these corals are called, with its lower part modified into a stem 
free from individual polyps; while the next figure exhibits a representative of 
another family, the sea-pens ( Pennatulidai ), which also form stocks divided into 
a polyp-bearing area and a stem resting on the sea-bottom. In one of the simplest 
forms of the sea-pens ( Veretillum ) the upper part is simply surrounded by polyps, 
and the lower a cylindrical stalk. A stock of this last-named type may lie for two 
or three consecutive weeks like a wrinkled turnip at the bottom of an aquarium, 
with all its activities suspended; no individual polyp appears; no food is taken in, 
and the circulation of water, necessary for the life of the stock, does not take 
place. After a time, however, the fine pores begin to take in water again, the 
surface becomes smoother, and gradually, as the individual polyps appear and 
stretch out their tentacles, the colouring of the whole stock becomes more vivid 
and more delicate. The stock lengthens and thickens, and the white crowns of 
tentacles stand out in dazzling contrast to the red of their bodies and of the 
