49 8 



CCELENTERA TES. 



the central axis. Monoxenia and similar animals are considered simple, because the 

 repeated organs develop similarly and simultaneously, and are comparatively few 

 in number. The mouth, too, is circular. In many other polyps, however, the 



regularly radiate type is slightly departed 



__8 from ; the mouth, instead of being round, 



forming a long slit, while there is a tend- 

 ency for the originally radiate animal 

 to become bilateral. From this account of 

 a simple polyp, it is easy to understand 

 what kind of animal it is which makes 

 coral ; and our readers, if they have not 

 already done so, will give up speaking of 

 " insects building up the coral - reefs." 

 It is, however, by no means all such 

 polyps that form coral, nor do those 

 which form it produce it always in the 

 same way. Numbers of polyps, such as 

 the beautiful sea-anemones, never produce 

 any hard substance, but remain soft and 



delicate, though dangerous, at least to 

 small animals, because of their stinging- 

 cells. Many of these soft sea-anemones 

 are highly specialised creatures, as may 

 be seen from the coloured Plate; but 

 those which secrete coral are generally 

 simpler, and smaller, and grow in vast 

 colonies. It is the accumulation of all 

 their little contributions of coral which, 

 in the process of time, build up islands or 

 even continents. In regarding coral 

 animals as reef -builders, we may leave 

 out of account, as unimportant, those 

 which form hard spicules within their 

 bodies, and consider only those which 

 perform most of the work. Imagine a 

 crowd of small animals like sea-anemones 

 fixed to a rock, each one secreting a layer 

 of carbonate of lime between itself and 

 the rock, and this layer becoming thicker 

 and thicker till each polyp rises on a 

 little pedestal. There is probably a race 

 between them, as there is among trees in a forest, which shall reach the highest 

 to get most of the food as it passes by on the currents in the water. Now, it is 

 obvious that a crowded colony of polyps like this would in a short time add a 

 thick layer of solid carbonate of lime to the rock on which they first settled, and 

 this is, in brief, the principle of reef -building. As a matter of fact, however, it is 



Monoxenia darwini (magnified). 



L, Longitudinal section, on the left, through one of the 

 interseptal cavities, on the right through one of the 

 partition walls ; M, Transverse section through the 

 line m n; N, Transverse section through the line 

 s b t ; O, The eight-lipped mouth-aperture, with the 

 bases of the arms ; a b c o, Principal axis ; p, 

 Pharyngeal cavity ; g, Digestive cavity ; k, Divisions 

 of the digestive cavity ; to, Radial septa or walls 

 dividing up the' digestive cavity ; e, Masses of eggs ; 

 u, Mesenterial filaments ; /, Masses of muscle and 

 connective tissue. 



