water. Distichopora is confined to shallow water. The genera Errina, Cryptohelia and Conopora 

 are confined to deep or very deep water, (204 — 1633 metres). 



The o-enus Distichopora is undoubtedly very common in the shallow waters of the Malay 

 Archipelago and is probably represented by only one variable species l ). Only one specimen 

 however was included in the collection sent to us and this presented no features of special interest. 



The crenus Stylaster however is represented by a large number of specimens from depths 

 ranging from o — 1633 metres, and it is in this genus that we have found the greatest difficulty 

 in assigning specific names to the specimens. 



Stylaster is one of those genera of zoophytes which, like Millcpora, Tubipora, Madre- 

 pora etc. have a wide geographical distribution in the shallow warm waters of the tropical 

 belt. It exhibits a great degree of variability in its form and habit due, probably, to the 

 development of considerable powers of adaptation to the variable conditions as regards strength 

 of current, light, food supply and other features of the environment in shallow tropical waters. 

 To divide the specimens of this genus into specific groups becomes increasingly difficult as 

 our knowledge extends. 



The consideration of the fact that the individual colonies of these genera usually occur 

 in great numbers and that there is nothing, so far as we are aware, to prevent constant cross 

 fertilisation in shallow water, renders it probable that the shallow water specimens of each of 

 these genera throughout the world represent only one true species. It has been shown that in 

 the case of Jlfillepora ') and Tubipora 3 ) at least the anatomical structure of the colonies 

 supports this conclusion. 



Stylaster however differs from some of the other genera of tropical zoophytes in the 

 fact that it extends from very shallow into very deep water and it is interesting to note that 

 the difficulty of arranging the deep sea specimens into specific groups is much less than in the 

 case of the shallow water specimens. It would not be surprising if it were ultimately proved 

 that in the case of Stylaster isolation in deep sea localities has led to the differentiation of 

 distinct species, in much the same way as isolation upon islands has led to the differentiation 

 of species of terrestrial animals. In considering the systematic arrangement of any genus of 

 zoophytes we must endeavour to distinguish between characters that are most probably modified 

 by the immediate conditions of the environment and characters which are more stereotyped or 

 fixed. In the genus Stylaster for example the shape assumed by the hydrophytum as a whole, 

 its colour, its delicacy or robustness, the position of the calicles, the general texture of the 

 surface etc. appear to be characters which are profoundly modified by the conditions of the 

 environment. On the other hand the position of the ampullae, the character of the styles, the 

 structure of the cyclosystems and many of the characters of the soft parts are probably less 

 liable to modification by the direct action of the conditions of the environment. In the present 

 state of knowledge and in the absence of any record of experimental work on the growth of 

 corals the conclusions we reach may not prove to be satisfactory but we feel that it is only in 



1) S. J. HlCKSON, Proc. Roy. Dublin, Soc. VII, 5. 



2) S. J. Hickson, On the species of the genus Millepora. Proc. Zool. Soc. IÏ 



3) S. J. Hickson, A. naturalist in N. Celebes 1S89, p. 129. 



