THE EIGHT-RAYED POLYPS 



3487 



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, nevertheless, 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. 3486 shows an alcyonarian, as these corals are called, with its 

 lower part modified into a stem free from individual polyps; while the figure below 

 exhibits a representative of another family, the sea-pens {Pennatulida) , 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 coloring 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 con- 

 trast to the red of their bodies and of the common trunk. 

 The foot swells out like an onion and becomes trans- 

 parent, curves, and sinks into the sand; and the stock, 

 which during its period of inactivity lay prone on the 

 ground, assumes an erect position. In these sea-pens 

 the parts of the stock may be compared with the parts 

 of a feather, the whole being bilaterally symmetrical, 

 and the single polyps being carried on the leaf-like 

 lateral appendages of the stem. The sexual animals, 

 which are provided with all the organs necessary for a 

 polyp, take in the food and reproduce themselves. The 

 other less perfect brethren called zooids, although more 

 or less resembling these, have remained at a lower 

 stage of development, are smaller, and have neither 

 tentacles nor reproductive organs. These zooids ap- 

 pear to perform only one function, namely, to pump 

 water through the body of the stock. In this 

 important work the higher individuals no doubt assist, 

 as indeed in most alcyonoid stocks they alone must 

 do the work, there being no such specialized pumping 

 polyps. 



A SEA-PEN, Pteroides spinosa. 



(One-fourth natural size.) 

 a. A polyp (somewhat magnified^. 



