PERCY SLADEN TRUST EXPEDITION 



Flabellum, characterised by the absence of a "Randplatte" or "edge-zone," Bourne con- 

 siders the apparent theca as " really a basal structure which has grown upwards to form a 

 calyx " and hence agrees with von Koch's view that it is really the epitheca. As far as 

 the colonial Astrseids are concerned the term epitheca may, for the present, be confined to 

 the thin, usually foliated lamina seen at the edges of the corallum. 



A study of the arrangement of the mesenteries in the polyps has shown that it is 

 highly misleading to regard the septal cycles, as seen in the dry corallites, as representing 

 the true succession. In genera characterised by the hexameral arrangement of the mesen- 

 teries as in my Group I, it often happens that the secondary septa in the entocoeles of the 

 secondary mesenteries meet the columella, thus simulating the true primary septa ; in 

 other cases the secondaries may be quite as narrow as, or even narrower than, the tertiary 

 septa. The true order of succession of the septa can thus be ascertained only in relation 

 to the mesenterial couples. No such arrangement of the septa into orders can be recognised 

 in the genera belongi ng to my Group II, which have lost the hexameral disposition of the 

 mesenteries ; in these some of the subsidiary septa may meet the columella and some of 

 the principal septa may not ; if a cyclical arrangement be recognised in the dry corallites, 

 it is always a later formation, the numbers of the cycles and of the septa composing them 

 being subject to so great variation that they are of little value for purposes of classi- 

 fication. 



The above is part of a general indefiniteness in the arrangement of the skeleton. It 

 is not surprising since the corallum lies entirely outside the external ectoderm, the part 

 most open to the free play of the environmental conditions and most liable therefore to be 

 modified by changes in these conditions. This variation is so considerable that a system 

 of classification based on the corallum alone can scarcely be a sure guide to the actual 

 relationships of the animals, for a proper knowledge of which it is essential to make 

 a comparative study of the morphology of the polyps. Bourne, on the other hand, in 1888 

 regarded any such attempt as futile : " every fresh form that is examined convinces me 

 that the expectations formed of founding a new classification of the Madreporaria on the 

 anatomy of the polyp are to meet with disappointment. There is singularly little variation 

 in the forms hitherto examined. Hence I believe that a re-modelled classification must 

 depend on a much more intimate study of the structure of the corallum than has hitherto 

 been attempted." (12, pp. 44 — 45.) 



As a result of my study of a limited group of the Astrseidee, it is clear that a thorough 

 re-casting, not only of the species but also of the genera concerned, is necessary. I am 

 further led to suggest that only by a comparative study of both the polyps and their 

 skeletons will a scientific classification be evolved. In the allied group of Actiniae the 

 importance of the internal characters of the polyps in classification has been fully 

 recognised since R. Hertwig's work on the "Challenger" Actinians. 



As an instance of the errors that may be made in basing a classification of corals 

 solely on the hard parts, the case of Orhicella, as later extended by Gardiner, may be 

 cited. This genus has to disappear in the light of the new relationships revealed by 

 a study of the polyps of the species included in it. For 0. minikoiensis Gard. — which 

 has been found to be identical with Heliastrcea heliopora, Ed. and H. — a new genus has 



