6 CORALS 



longed stud}^ of them is, as a matter of fact, insuperable. 

 And it is the same with many other zoological words and 

 expressions, because, as we become better acquainted with 

 the vast number of species and varieties of animals and 

 plants which exist in the world, the more clearly do we 

 realise that our systems of classification and the frontiers 

 we establish between one group and another are artificial 

 and unnatural. If we knew all that could be known about 

 animals living and extinct we should find that there are 

 no boundaries separating one group from another, but a 

 continuous series of forms showing tendencies to a great 

 increase in numbers in certain parts of its course or in certain 

 periods of time. 



It is not surprising, therefore, that as our knowledge 

 expands our system of groups is changed, and with the 

 change there comes inevitably an alteration in the meaning 

 of words. Such a change of meaning in the word Coral has 

 taken place during the sixty-five years that have elapsed 

 since (in 1857) Milne-Edwards published the important 

 treatise entitled Histoire natnrelle des coralliaires on polypes 

 proprement dits. In this great work the class of the 

 Coralliaires was defined as " Radially symmetrical animals 

 with the following characters : (i) A centrally placed mouth 

 surrounded by tentacles and no true anus ; (2) the body 

 provided with a single system of cavities of which all parts 

 communicate freely with one another and with the exterior ; 

 and (3) the organs of generation are situated in the general 

 cavity of the body." With this definition of the Coralliaires 

 the distinguished French author included in the treatise not 

 only the Alcyonaria and Zoantharia that form coralline 

 structures of calcium carbonate, but also the whole group of 

 the Sea-anemones, the spongy Alcyonidae, the Order of the 

 Sea-pens (Pennatulacea), and several other forms that are 

 not, at the present day, usually called Corals. On the other 

 hand he excluded from his treatise all the coralline Algae, 

 Protozoa, and Polyzoa, and if he had been in possession of 

 the knowledge we have gained since his time he would also 

 have excluded, on the strength of his definition, such im- 

 portant corals as Millepora and the Stylasterina. 



