THE SIMPLEST MANY-CELLED ANIMALS 337 



There are many interesting minor points of structure omitted from 

 these brief notes. For supplementary work, consult Linville and 

 Kelly's "General Zoology," Chapter XIX. 



Formation of Coral Skeletons. On every piece of coral 

 which has not been corroded by water, there may be seen 

 numerous cup-like depressions, each with many radial 

 partitions. In each of these cups there was once seated a 

 coral-animal, or coral-polyp, which secreted the cup. Im- 



FIG. 105. An Astraea coral colony, with living animals on surface of hemi- 

 spherical skeleton. Such a colony originates from a single egg which 

 develops into a polyp capable of budding to form a colony. (After Dana.) 



agine a sea-anemone able to secrete a calcareous skeleton 

 around itself, and also to secrete a partition of the same 

 material between the two membranes of each pair seen in a 

 transverse section, and then you can understand the relation 

 of a coral-animal to its surrounding cup. 



The fact that a piece of coral shows many cups is explained 

 by the multiplication of individuals by budding. A coral- 

 animal which develops from a fertilized egg-cell settles 

 down on a rock or on skeletons of pre-existing corals and begins 

 to secrete a skeleton around itself. Buds are formed, which 

 do not become detached as in Hydra, and these begin to 



