HYDRA 



275 



The coral polyp does not differ essentially from Metridium. 

 But it has the habit of secreting lime at its base, so that, in 

 com-se of time, a high cup is built up. The top of the cup 

 bears radial septa,' which are laid down in the interspaces 

 between the fleshy partitions such as are found in j\Ictridiian. 

 Since most corals, like hydroids, bud freely, and since every 

 bud secretes coral at its base, an extensive and complicated 

 limy mass may be produced. This is the way in which the 

 brain corals and branching corals are formed. 



Fig. 265. — Coral r-up of Manicina. Nat. size. Photo, by W. H. C. P. 



The only coral of the northeastern United States is Astrangia 

 dance, which occurs north to Cape Cod. It is a beautifully trans- 

 parent species, and forms thin encrustations of limy matter (Fig. 

 266). 



The Sea Walnuts or Ctenophora are a small group of exclu- 

 sively marine organisms that are usually placed in the group 

 Cnidaria, but differ very widely either from the Hydra or the 

 Scyphozoa. The mouth lies centrallj^ at the lower ]3ole of the 

 barrel-shaped animal, and is usually guarded by one or two 

 tentacles. The food-cavity runs through the axis of the body, 

 enlarging near the upper pole. From this enlargement a 



'Fig. 265. 



