396 COELENTERATA ANTHOZOA chap. 



standing the difficulty of distinguishing the species, the genus 

 itself is quite well defined. The calices project slightly from the 

 surface of the branches and contain six septa, of which the pair 

 that is parallel with the axis of the branch is the strongest. 

 This strong pair of septa can usually be well seen when a slender 

 branch of a Madrepore is examined by a lens by transmitted 

 light. At the apex of each branch there is a terminal zobid and 

 in the skeleton an apical calyx. The terminal zooid is (in some 

 species at least) different from the lateral or radial zooids. The 

 former is radially symmetrical and has six long equal digitiform 

 tentacles, the latter have usually twelve tentacles, of which six 

 are larger than the others. These tentacles alternate, but tliey 

 are so arranged on the disc as to give a distinctly bilateral 

 appearance to the zooids. 



The colour of the West Indian Madrepores appears to be 

 entirely due to Zooxanthellae (pp. 86, 125). They are lighter 

 or darker shades of brown, sometimes becoming green, yellow, 

 or orange. On the Australian barrier reef and other reefs of 

 the eastern seas the growing points of the branches are variable 

 and often brilliantly coloured, emerald green, violet, or red ; 

 giving some of the most wonderful colour effects for which the 

 reef pools are famous. The cause of these brilliant apical 

 colours has not yet been ascertained. 



The genus is found in shallow water of all seas of the tropical 

 belt except on the western side of the continent of America. 



Montipora. — In this genus the calices are small and situated 

 in depressions in the coenosteum, and there are six, sometimes 

 twelve, septa of approximately equal size. There is no terminal 

 calyx at the apex of the branches. This is a genus of very variable 

 form and wide distribution in all tropical seas except on the 

 shores of the Atlantic Ocean. 



Turhinaria. — This genus is usually cup-shaped or foliaceous 

 and twisted in form. The septa may be six to thirty in 

 number. Some of the species of this genus attain to a very 

 great size in favourable localities. There is a specimen in the 

 British Museum that is 16 feet in circumference and weighed, 

 when dried, 1500 lbs. 



Fam. 2. Poritidae. — The corallum is usually encrusting, 

 foliaceous, lobed or tufted, rarely dendritic. The whole skeleton 

 is built up of a system of trabeculae and stout cross bars, and in 



