662 PUOF. MiJsrcHiN and dr. reid on the [June 16^ 



Historical Review of the Qioestion. 



Grant (1826 *) pointed out that in certain sponges the skeleton 

 is calcareous, consisting of carbonate of lime, and exhibiting no 

 trace of phosphate of lime. In Sjwngia (Grantia) compressa he 

 described the rays of the triradiates as " hollow within, shut at 

 their free extremities, and having no superficial openings ; but 

 their internal cavities communicate freely at theii- point of junc- 

 tion and form there a small central reservoir." The monaxons 

 are described as " distinctly tubular and shut at both extremities. "^ 

 Of the triradiates of S. {Leuconia) nivea he remarks, " their 

 internal cavities are very distinctly seen." 



Schmidt (1862), on the othei' hand, included the spicules of 

 calcareous sponges in that category of spicules in which both 

 central canal and lamination were entirely wanting. He considered 

 it beyond doubt, however, that organic substance takes part in 

 building up calcareous sponge-spicules, since heating produces, 

 small vesicles in them. 



Bowerbank (1864) described the effects of heat on siliceous 

 and calcareous sponge-spicules, and concluded that the latter 

 contained so great a proportion of calcareous matter as to prevent 

 their disintegration by heat. He stated the concentric sti^atifica- 

 tion to be visible in the transverse fractures of any spicule^ 

 calcareous or siliceous. 



Kolliker (1864), on the other hand, was unable to find any 

 stratification or other internal structure in calcareous sponge- 

 spicules, and considered it doubtful if they contained any organic 

 matter, since no residue was detected if the spicules were dissolved 

 with acids. He described the spicule-sheaths in '■^ Narcloa spon- 

 giosa " (probably a synonym of Clathrina contorta, vide Minchin, 

 1898, p. 533, footnote, and P. Z. S. 1905, ii. p. 17). In siliceous 

 sponge-spicules, however, Kolliker observed and described, in 

 detail, the axial filament and the stratification of the silica. 



Lieberkiihn (1865) observed in the gastral rays of the quadri- 

 radiates of Leucosolenia "a fine layer of the contractile substance, 

 which pi-otrudes between the ciliated cells and either envelopes 

 the spicule partially oi- completely as a fine layer, or only surrounds, 

 the foot of it as a stronger thickening (Anhaufung)." These 

 sheaths were left behind when the spicule was dissolved with 

 acetic acid ; they were considered by him as retractile. From the 

 description it is evident that the structures observed by Lieberkiihn 

 were I'eally the cellular sheaths or gasti'al actinoblasts enveloping 

 the projecting gastral rays, and not the true spicule-sheaths. 



Carter (1869) stated that the spicules of Grantia ciliata differ 

 from those of siliceous sponges in lacking a central canal ; bvit in 

 his later note of the same year, he modified this statement and 

 admitted that something like a central canal may often be seen 

 towards the base of the straight arm of a triradiate ; while for 

 the most part there is no trace of a central canal nor of the 



* For bibliogi-aphy, see pp. 674-5. 



