MADREPORIANS. 



17 



title, because any account of the Radiates, from which so impor- 

 tant a group as that of the corals was excluded, would be very 

 incomplete. 



This pretty coral of rig. 10. 



our Northern waters is 

 no reef-builder, and does 

 not extend farther south 

 than the shores of North 

 Carolina. It usually es- 

 tablishes itself upon brok- 

 en angular bits of rock, 

 lying in sheltered creeks 

 and inlets, where the vio- 

 lent action of the open sea is not felt. The presence of one of 

 these little communities on a rock may first be detected by what 

 seems like a delicate white film over the surface. This film is, 

 however, broken up by a number of hard calcareous deposits in 

 very regular fbrm (Fig. 20), circular in outline, but divided by 

 numerous partitions running from the outer wall to the centre of 

 every such circle, where they unite at a little white spot formed by 

 the mouth or oral opening. These circles represent, and indeed 

 are themselves the distinct individuals (Fig. 17) composing the 

 community, and they look rig. 17. 



not unlike the star-shaped 

 pits on a coral head, formed 

 by Astraeans. Unlike the 

 massive compact kinds of 

 coral, however, the indi- 

 viduals multiply by bud- 

 ding from the base chiefly, 

 never rising one above the 

 other, but spreading over 

 the surface on which they 

 have established them- 

 selves, a few additional individuals arising between the older 

 ones. In consequence of this mode of growth, such a commu- 



Fig. 16. Astranpia colony; natural size. 



Fig. 17. Magnified individuals of an Astrangia community in different stages of expansion. 

 3 



