HYDROZOAN CORALS 145 



sterina. In many text-books of zoology they are still grouped 

 together to form the Order Hydrocorallinae, but although 

 they have in common the two characters of dimorphism and 

 a massive calcareous corallum, the structure of the polyps 

 and of the reproductive bodies suggest that the resemblances 

 between the two groups are due to convergence rather than 

 to genetic affinity. 



The Order Milleporina is constituted for only one genus 

 — Millepora — which has a wide distribution in the warm 

 shallow waters of the East and West Indies. It was A. 

 Agassiz in 1859 who first proved that the correct position 

 of the genus is in the class Hydrozoa, but Moseley's brilliant 

 researches during the voyage of H.M.S. Challenger in 1876 ^ 

 provided us with the first correct account of its general 

 structure. 



The corallum assumes many varieties of form. Some- 

 times it consists of thick massive plates, sometimes it is 

 coarsely branched or becomes profusely ramified. These 

 differences in form seem to be associated with differences 

 in the conditions of the immediate environment and cannot 

 be used as characters for specific distinctions. 



The special characters of the corallum can be easily 

 recognised with the help of a simple magnifying glass. The 

 surface is perforated by a very large number of pores, and 

 these pores are of two sizes, the larger or gasteropore 

 (about 0-25 mm. in diameter) and the smaller or dactylo- 

 pores (about 0-15 mm. in diameter) (Fig. 66). 



When examined in sections these pores are seen to lead 

 into delicate tubes which pass radially towards the centre 

 of the branch, and each tube is divided into a number of 

 chambers by very thin transverse partitions called the 

 tabulae (Fig. 67, Tab.). Between the tubes the corallum is 

 seen to be perforated by a dense plexus of branching canals. 

 On account of its porous texture Millepora was named by 

 Rumphius Lithodendriim saccharacemn album, or the White 

 Sugar Coral. 



The corallum is therefore perforate, tabulate, and pro- 

 vided with dimorphic pores. 



^ H. N. Moseley, Challenger Reports, vol. ii., 1881. 



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