MADREPORARIAN CORALS 25 



of cup-like depressions for the polyps, and in the other, 

 those that consist of a single cup. The former are called 

 " Colonial " corals and the latter " Solitary " corals. 



If the corals are then examined with a magnifying glass 

 it is found that, in some, the walls of the cups and other 

 structures are porous, and in others they are solid. The 

 former are called " Perforate " and the latter " Imperforate " 

 corals. 



These two methods of grouping, however, are not similar, 

 for in both the solitary and in the colonial groups there are 

 examples of perforate and imperforate corals. 



An examination of the base of a branch that has been 

 broken off a large corallum also shows that the tubes which 

 lodge the polyps are in some cases divided by transverse 

 partitions or " Tabulae," as in Millepora and Heliopora, 

 and in other cases are not so divided, and thus we can 

 speak of corals that are tabulate and corals that are not 

 tabulate. 



All of these characters of the corallum may be of import- 

 ance in the description of the corals and in their classifica- 

 tion into families and genera, but it has been found that no 

 one of them affords a sufficiently trustworthy character for 

 the arrangement of the corals into large groups which are 

 intended to express genetic relationships. 



For this purpose reference is made to the characters 

 that are showai by the anemone-like polyps which construct 

 the coralla, such as, the number and form of the tentacles, 

 the arrangement of the mesenteries, and the methods of 

 gemmation and fission. It seems probable that these 

 characters have undergone less change in the course of 

 evolution than the characters of the coralla, and that they 

 are, therefore, more trustworthy guides to genetic afftnities ; 

 but even these characters cannot alone serve the purpose 

 of a sound classification unless taken in conjunction with 

 the characters of the corallum. 



In order that the student may become acquainted with 

 certain technical terms that it is necessary to use in the 

 description of the Madreporarian corals, it is best to 

 examine, in the first place, the structure of a specimen 



