MADREPORARIAN CORALS 



73 



but the researches of Duerden have shown that the process 

 is only a special form of gemmation which he calls " fissi- 

 parous gemmation." 



The appearance of a living colony of a West Indian 

 Siderastraea has also been fully described by Duerden. ^ 

 According to this author the small expanded polyps (5-6 mm. 

 in diameter) are outlined by a narrow polygonal groove of 

 a hghter colour than the rest of the polyp wall. This 

 groove corresponds with the upper limit of the common 

 calicinal wall between the polyps. The expanded polyps 





'■-^^■* '^ 



Fig. 27. — Siderastraea siderea. A small portion of the surface of a specimen 

 from the West Indies, showing the calices, septa, and synapticula. x 6 diams. 



rarely assume a cylindrical form with a ifattened terminal 

 disc, like most coral polyps, but exhibit merely a dome- 

 like elevation of the walls over the calyx (2 or 3 mm. high). 

 In contraction they are not covered over by a fold of the 

 body wall, but, as in Fungia, the tissues and the tentacles 

 sink down as far as possible into the spaces between the 

 septa. The tentacles are wide apart and occupy a broad 

 band round the oral disc. When fully expanded they 

 consist of a broad basal part which, in most members of the 



^ J. E. Duerden, " The Coral Siderastraea," Carnegie Publications oj 

 Washington, 1904. 



