an Extraprotoplasmic Secretion of the Polyps! 161 



viz., the spicule-sheath. At the same time, Bourne contends that 

 the spicule is entoplastic in formation whilst the Madreporarian 

 coralkim is exoplastic. To be consistent, both the spicule and the 

 corallum would have to be regarded as formed either within living 

 protoplasm or outside it, but spicules could not be viewed as intra- 

 protoplasmic products whilst assuming the extraprotoplasmic origin 

 of the corallum. 



Duerden (3) held that the organic basis of the corallum of 

 Siderastrea yadians is a " secretion " of the calicoblastic layer of 

 ectoderm to which it is closely adherent (pi. 8, fig. 45) and is "a 

 homogeneous, mesoglaea-like matrix within which the minute cal- 

 careous crystals forming the skeleton are laid down " (p. 34). 

 Since he refers to the skeleton as " ectoplastic " in origin (p. 113), 

 it is evident that he agi-ees with Bourne in the view that the 

 organic matrix was not part of the living tissues when calcareous 

 matter began to be deposited in it. But in the account of these 

 authors there is no more evidence to show that, in the Madre- 

 poraria, the organic ground substance or "colloid matrix " (2, p. 539) 

 was non-living at every phase of skeleton formation than that the 

 areas of the scleroblasts of the Alcyonaria in which the deposition 

 of spicular matter took place had not, at least at the initial stages 

 of this process, formed part of the living protoplasm. 



Further if, in the Madreporaria, the calcareous matter were 

 deposited outside the living calicoblastic ectoderm, it is difficult 

 to understand how the manifold patterns of eoralla so charac- 

 teristic of this gi'oup of organisms can have been built up*. But 

 if the matrix in which carbonate of lime is laid down is part 

 of the living calicoblastic sheet, it follows that the protoplasm 

 must regulate the arrangement of the calcareous matter into the 

 various skeletal types which, in large measure, maintain their re- 

 spective form independent of changes in environmental conditions. 

 Similarly, the formation of the various kinds of spicules of the 

 Alcyonaria can be adequately explained only if calcareous deposition 

 takes place within living protoplasm, and indeed. Bourne has drawn 

 attention to the phenomenon that " the spicules of the Alcyonaria 

 show a definite and complex crystalline structure, the details of 

 which are, indeed, moulded upon and dominated by an equally 

 complex organic matrix..." (2, p. 517). 



The intraprotoplasmic origin of spicules in the Alcyonaria might, 

 without difficulty, be ascertained since sections can be made with- 

 out decalcification, whereas in Heliopora and the Madreporaria 

 possessing massive eoralla, satisfactory sections are possible only 

 after decalcification, and in this condition the skeleton may appear 



* In explanation of this phenomenon, Bourne suggests that "the general 

 arrangement of the fasciculi of crystals is dominated, in some manner of which we 

 are ignorant, by the living tissues which clothe the corallum " (2, p. 539). 



VOL. XIX. PART IV. 12 



