MADREPORARIAN CORALS 53 



anemones and Madreporarian coral polyps, but in the 

 Astraeid coral polyps that divide by fission the directive 

 mesenteries are usually absent. Duerden, who first called 

 attention to this fact, considered that the absence of directive 

 mesenteries in the polyps of an Astraeid colony could be 

 taken as a sign that the method of reproduction was by 

 fission, and vice versa that the presence of the directive 

 mesenteries was a sign that the method of reproduction was 

 essentially one of gemmation. There are some exceptions, 

 apparently, to this interesting and important generalisation, 

 for in the genera Cladocora, Stephanocoenia, and Solena- 

 straea, which are apparently fissiparous, the directive 

 mesenteries occur, and in two species of Favia investigated 

 by Matthai the polyps that arise by gemmation do not 

 possess directive mesenteries. 



GoxiASTRAEA. — In the genus Goniastraea (Fig. 18), 

 another widely distributed coral on the tropical reefs, the 

 calices are so crowded together that the thecal walls are 

 actually in contact, the common coenosteum being ap- 

 parently absent. As a result of this crowding the calices 

 have lost their round contour and become angular, but 

 they do not form a hexagonal pattern, some being triangu- 

 lar, some roughly quadrangular, and others pentagonal or 

 irregular. 



Among the more irregular forms a few calices may 

 usually be found with a constriction in the middle which 

 shows that the usual method of reproduction is by fission. 



In the characters of the septa and in some other respects 

 the genus Goniastraea is very similar to Favia. 



We have seen that in Favia and Goniastraea some of 

 the calices become elongated and then constrict to form 

 two calices. In other genera the elongation takes place, 

 but the constriction is delayed so that the calices assume 

 the form of long straight or sinuous grooves, provided on 

 each side with rows of septa and separated by ridges from 

 similar grooves representing the neighbouring calices. The 

 extreme forms of this modification are seen in the group 

 of Astraeids commonly known as the Brain corals, from the 

 fact that these sinuous calices give the rounded surface of 



