134 SYDNEY J. HICKSON, M.A., D.SC, F.R.S., P.Z.S. 



correct definition of a species, which would be agreed to by all system- 

 atic zoologists, our task might be less difficult than it is ; but, as 

 matters stand, the conception of what is a species of one worker is so 

 different from that of another that there is constantly going on a see- 

 saw of construction and destruction of new species in our systematic 

 literature. 



I do not propose to attempt to define the conception "species" in 

 Coelenterates, but I think that all zoologists would agree that, if a form 

 which is known as species A were proved to give rise to an embryo 

 which grew into a form which had hitherto been known as species B, 

 the two forms would have to be merged into one species with one 

 specific name. Similarly, I imagine that all zoologists would agree 

 that if a coral known as species X changed in the course of its life- 

 history into a form known as species Y, then the forms X and Y should 

 be regarded as one species and retain only one name. In the absence 

 of any experimental proof that the embryo of one so-called species of 

 coral gives rise, under any circumstances, to another so-called species, 

 or that one so-called species changes in the course of its life history 

 into another, it is necessary to examine, with very great care the 

 anatomy of the soft parts as well as the skeletal structures, in order to 

 determine whether it is possible or even probable that such changes 

 actually occur in nature. If we find, then, that the polyps or repro- 

 ductive organs of a coral with one form of growth are essentially 

 different from those of another form, we may consider there is good 

 reason for believing that such changes do not occur and the species 

 founded on the skeletons are good ; but if, on the other hand, the 

 polyps, reproductive organs, and other characters of the two forms are 

 essentially the same, then there is reason for believing that the species 

 founded on skeletal characters may not be good. 



Before proceeding further with this discussion of the characters 

 which may be used for distinguishing species in Coelenterates, it may 

 be well to describe briefly the general results of my observations on the 

 genus Millepora. This genus stands quite by itself among living corals. 

 No one genus of the other Hydrocorallines can be confused with it, 

 both the living tissues and the hard skeletal parts being perfectly 

 distinct. It is widely distributed through the tropical seas, occuring in 

 the Red Sea, Indian Ocean, Malay Archipelago, Tropical Australian 

 waters, Pacific Ocean, and in the seas of the West Indies. It is essen- 



