308 Mr. H. J. Cavtev on the Strom atoporidas. 



sponge, as is often the case in specimens from the West Indies, 

 it is generally rubbed off like the corresponding parts in Tuhi- 

 pora musica. The process of incorporating the spicular 

 elements into calcareous fibre seems analogous to that which 

 takes place in the vitreous hexactinellid sponges, wherein the 

 mould of the spicule may be brought back hj some solvent, 

 viz. an acid in the former and an alkali (as Mr. Sollas has 

 shown) in the latter. In Corallium ruhrum a transverse 

 section ground down to extreme thinness will well show what 

 was stated by Donati so long ago. 



Here and there again, on the surface of the " superficial 

 structure," may be seen the calicular spaces of the full-formed 

 poly])ite and those of the zooid respectively, more or less irre- 

 gularly scattered about, more or less thickly, more or less in 

 number, the one or the other, sometimes in groups, as in the 

 species from Tahiti figured by Mr. Moseley, of which I also 

 possess a specimen from the West Indies. 



Occasionally, as before stated, the surface of the " super- 

 ficial structure " presents the grooved venation indicative of 

 the vascularity of the " proliferous membrane;" while for the 

 most part this only becomes visible after the surface-layer 

 has been jiicked off, as before stated, when it may be seen 

 sunk into the outer layer of the " middle structure." 



Splitting now the branch longitudinally, both the "middle" 

 and the " axial structure " are brought into view, when the 

 former, of course, presents the same features as in the trans- 

 verse fracture ; while the " axial," which in the transverse 

 fracture only appears as a cribriform surface, is now found to 

 be composed of longitudinal tubes in juxtaposition, more or 

 less interrupted by tabula;, and more or less pierced with holes, 

 by which they communicate with each other. 



In most instances also, as before stated, the grooved vena- 

 tion, which represents the larger vessels of the " proliferous 

 membrane " or hydrophyton, is concealed beneath the super- 

 ficial layer ; but as the branch grows by additional layers to 

 its surface, it is evident that this layer must be thin before it 

 can be thick, and that therefore, if the specimen meet with its 

 death or be seen when the surface-layer is thin, the vascu- 

 larity will be visible to the naked eye, and vice versa. Hence 

 the absence of the grooved venation on the surface in some, 

 but not in all, specimens may receive this explanation both in 

 the recent and in the fossilized structures. 



This seems to be the proper place to notice the differences 

 that exist between a coral (Actinozoic or Hydrozoic) and a 

 sponge, whicli may be thus described, viz. : — 



A coral grows from an embryo which develops one animal, 



