CORALS. 



5" 



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 same 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 (Pennatulidce), 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 



