162 Mr Matthai, Is the Madrejjorarian Skeleton 



as though formed outside the living tissues. A further difficulty 

 with regard to the Madreporaria is that, except perhaps at the 

 growing points, the skeleton would secondarily lose its intraproto- 

 plasmic character and appear to be external to the living tissues by 

 having displaced most of the protoplasm in which it was deposited, 

 just as the discrete condition of fully developed Alcyonarian 

 spicules is due to the increase of calcareous matter at the expense 

 of the protoplasm in which it was formed. 



From the above considerations it would appear to be highly 

 probable that von Heicler was right in regarding the Madreporarian 

 skeleton as formed within the calicoblastic protoplasm. Bourne 

 directs much of his criticism to von Heider's suggestion that the 

 striae in the calicoblastic layer (i.e., in the processes of attachment) 

 are calcareous fibres, but it is not improbable that, in the unde- 

 calcified condition, some of these processes of attachment might 

 be partially calcified. 



When thin sections of Astrseid coralla are examined under a 

 microscope, they frequently appear to consist of calcareous pieces 

 united by sutures resembling the " laminae " or " trabecules " of the 

 skeleton of Heliopora (1, p. 463, pi. 11, figs. 7 and 8) and the " tra- 

 becular parts " of the Madreporarian skeleton as figured by Ogilvie 

 (9, p. 124, figs. 13, 19, etc.). Each piece is composed of calcareous 

 strands radiating from a dark centre or line which, as Ogilvie sug- 

 gested, appears to be the organic remains of the protoplasm in which 

 the calcareous needles were laid down. There is some similarity 

 between these elements and the spicules of Tuhipora (7, figs. 9 

 and 10) which, according to Hickson, are not fused together but 

 dovetailed into one another as in the membrane bones of Mammals 

 (p. 562). The resemblance is also marked in the case of the scale- 

 like spicules of Plumarella (2, figs. 6 and 7) containing dark centres 

 from which calcareous fibres or rods radiate. 



It is difficult to gather from Bourne's account what he considers 

 to be the unit of skeletal structure in the Alcyonaria. Are 

 spicules such units* ? But spicules are not all homologous elements 

 since they are formed in protoplasmic areas containing one or more 

 nuclei and no limit can be set to their size in the various genera 

 (2, pp. 508-517), an extreme case being the scale-like spicules of 

 Primnoa and Plumarella, each of which is " formed by several 

 cells, or at least by a comparatively large coenocytial investment 

 containing many nuclei " (p. 510). Or, is a spicule a calcareous 

 piece which behaves like a single crystal when examined under 

 crossed Nicols? The same confusion prevails with regard to ske- 

 letal units in the Madreporaria — whether they are represented by 

 " fibro-crystals " (Bourne), "crystalline sjjhgeroids" (von Koch) or 



* Bourne applies the term spicule to "an entoplastic product of a single cell or 

 of a ccenocyte " (2, p. 504). The italics are mine. 



