244 RADIATES: POLYPS. 



or three inches in diameter, and most of them are much 

 smaller, although some are six inches high. The Bu- 

 nodes, Figure 497, is found among the rocks on the 

 coast of Maine. The most common kind on the north- 

 east coast of North America is the Fringed Actinia, or 

 Metridium, Figures 495, 496, 498. When fully ex- 

 panded, it is about four inches high and three inches 

 across the disk, and is a most interesting object for study. 



MADREPORES, PORITES, M^ANDRINAS, ASTR^EAS, 

 &c., OR MADREPORARIANS. 



These polyps are simple or compound, often exces- 

 sively branching, and they form coral in their walls, or 

 outer parts, in their radiating partitions, and often at 

 their base. The forms which the communities assume 

 are very beautiful and exceed ingly various, and are 

 among the most beautiful objects in zoological cabinets. 



The great group of Madrepores contains polyps which 

 have a definite number of tentacles, twelve or more ; 

 those called Porites, Figure 500, have the cells shal- 

 low, and not more than one twelfth of an inch in di- 

 ameter, and the coral in some cases branching, in oth- 

 ers massive, and always very solid. Massive specimens 

 of Porites are sometimes fifteen feet in diameter. In 

 the true Madrepores, Figure 499, the polyps do not 

 secrete coral at the base, and hence the cells of the 

 coral are very deep, and these corals spread and 

 branch into the most beautiful and varied forms, and 

 the polyp at the end of a branch, Figure 499, is al- 

 ways larger than the others. 



In the great group of the Astraeans the tentacles oc- 

 cur in multiples of six. Those of this group, called 

 Brain Corals, or Maeandrinas, have the surface covered 



