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TEXT-BOOK OF ZOOLOGY 



but below the level of the latter project freely into the body cavity, like 

 wings on the stage of a theatre. Each chamber is continued above into 

 a hollow, unbranched tentacle. The number of these tentacles (as in 

 the following group) is usually six or a multiple of this number. When 

 touched, the sea-anemone at once shrinks together to such an extent 



that the once flower-like creature now 

 resembles an inconspicuous lump (see 

 illustration, p. 421). 



In the Stone Corals (Madrejio- 

 raria), contrary to what we see in the 

 sea-anemones, the young animals pro- 

 duced by budding (see p. 469) remain 

 connected with the parent stock and 

 with each other. Hence arise colonies 

 Buch as we have already seen among 

 the Hydrozoa. As in the latter, we 

 here also meet with structures — gener- 

 ally in the form of stony calcareous 

 secretions — which supply the colony 

 with its necessary support. Each 

 coral stock, or corallum, takes its 

 origin from a single zoophyte, which 

 attaches itself to some solid object, 

 generally a stone or rock, and de- 

 velops into a small sea-anemone. 

 On the lower surface of its cylindrical 

 body the animal secretes a calcareous 

 plate (foot-plate), by which it is con- 

 nected to the surface of support. 

 Next, by further secretion of calcar- 

 eous material, a number of radiating 

 ridges (costse) arise from the foot- 

 plate, their outer ends being united 

 by an annular wall (the theca). The 

 "calcareous skeleton" thus developed 

 pushes the body of the polyp in front of it, as it were, whence it often 

 happens that thousands of the living animals are continuing to build 

 upon the branches of the corallum, whilst its lower parts have long since 

 died off. The immense reefs constructed by these minute delicate 

 animals in tropical seas, sometimes forming whole island groups and 

 mountain chains, are of great importance both geographically and 

 geologically. 



Longitudinal Section through a Sea- 

 Anemone (Diagrammatic). 



The section has been carried out in such a 

 manner as to expose on the left one of 

 the mesenteric ohambers (K.), and on the 

 right a mesentery (S. ) ; in addition to the 

 latter, four others are shown (S.l), which 

 project into the body cavity like the 

 winga on a stage : on the lower part of 

 the mesenteries eggs (E. ) have developed ; 

 Sch., oesophagus ; M., mouth ; A., ten- 

 tacles : into that furthest on the left the 

 body cavity is seen to extend. 



