SIX-RAYED POLYPS 



3479 



True Corals 



also differ much in size, but stand in regular rows divided by furrows, and serve 

 for attaching the colony, and also enable it to creep. The colony can even be 

 seen slowly climbing up and down small stones. The polyps have no septa in the 

 digestive cavity, the inner side being quite smooth; and the lower end of each is not 

 closed, but communicates with a forge cavity running along the whole colony, and 

 divided at regular intervals by partitions. 



From the foregoing observations it will be seen that in the soft divi- 

 sion of the Hexactinia, or six-rayed anemones, there are both single 

 individuals and colonies of individuals joined together to form stocks; and there is 

 also the same diversity in the skeleton-producing division the corals proper, 

 where we have both single individuals and stocks. Where- 

 as, however, in the soft division, the simple individuals are 

 the more numerous and the colonies comparatively rare, 

 among the corals the opposite is the case, the colony- 

 forming types presenting almost innumerable varieties. 

 This is not difficult to understand, since the soft anemones 

 cannot well form complicated colonies, whereas the skele- 

 ton-forming polyps, by combining their skeletons, can build 

 complicated structures, in order to raise themselves into 

 more advantageous positions. We have first, then, to con- 

 sider those corals which do not typically form stocks, but A SIMPLE CORAL, Thecocya- 

 remain at the stage of a simple sea anemone, only with a thus (natural size) . 

 rigid, calcareous skeleton supporting, and no doubt pro- 

 tecting, them in different ways. All the corals found in British seas are (with the 

 exception of the so-called tuft coral) single, and generally very small. As an 

 example of a regular, circular, solitary coral, we may take Thecocyathus cylindraceus, 

 the skeleton of which is shown in the illustration. The animal when expanded fills 

 up the central depression, but when, on expelling the greater part of the watery 

 contents of its cavity, it contracts, the whole body seems to sink into the hollow cup 



formed by its skeleton. In the 

 illustration we see only the outer 

 wall and the top of the ring of 

 septa, which are solid vertical 

 plates, rising up from the pedes- 

 tal secreted by the foot and radi- 

 ating outward in all directions. 

 Two other solitary corals are 

 worth describing, as they show 

 certain interesting specializa- 

 tions. Both of them may in- 

 crease by budding, that is, by 

 the method which, in colony- 

 forming corals, leads to the formation of stocks, if the buds remain attached to their 

 parents. When, however, solitary corals bud, the buds fall off, and lead solitary 

 lives like their parents. 



SCARLET CRISP CORAL, Flabellum (natural size). 



