ANATOMY OF MTJSSA AND EUPHYLLIA. 45 



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. 



Von Heider ( f Zeit. fur wiss. Zool./ xliv, p. 507) recognises 

 this, and attempts to found two new divisions, Euthecalia 

 and Pseudothecalia, on the characters of the corallum. 



His Pseudothecalia would include all those forms whose 

 theca is formed by fusion of the peripheral ends of the septa ; 

 his Euthecalia all forms in which the theca is a separate and 

 distinct structure (according to him, Astroides, and perhaps 

 Flabellum). His classification is based upon his account of 

 the structure of Astroides, which I cannot accept without 

 further evidence. It stands in direct contradiction to von 

 Koch's account of the development of the same genus. Von 

 Heider states that the corallum is clothed externally by a 

 single layer of ectoderm, mesoglcea, and eudoderm, the latter 

 layer abutting on the corallum. How then is the corallum 

 external to the polyp, as it undoubtedly is, if von Koch's 

 account of the development be true, and there is no reason to 

 doubt that it is ? Thus there is no proper " Randplatte/' no 

 extrathecal coelenteron, in Astroides, according to von Heider; 

 yet von Koch expressly states that the theca cuts the mesen- 

 teries in two, and divides the polyp into an outer moiety and 

 an inner moiety. Finally, in the " Euthecalia " founded on 

 Astroides, we learn from development that the theca is formed 

 from the fused ends of the septa; yet, by definition their theca 

 is a distinct structure. Unfortunately the specimens of 

 Astroides which I have examined were not well preserved 

 enough to admit of accurate observation. The external surface 

 of all was closely invested by a calcareous sponge, which had 

 in some cases apparently destroyed all traces of polyp external 

 to the corallum. Might not von Heider have been deceived 

 by the existence of a similar sponge on his specimens? 



In attempting to collect material for a new classification 

 from the observations already made, we must pay attention 



