664 PROF. MINCHIN AND DR. REID ON THE [Juiie 16, 



was stated to contain more organic substance than the younger, 

 I. e. outer parts. 



An entirely new epoch in the study of calcareous sponge- 

 spicules was inaugurated by the elaborate and exhaustive inves- 

 tigations of Ebner (1887), by whom and by Sollas, independently, 

 the peculiar crystalline nature of these spicules was discovered ; 

 namely, the fact that each spicule, whatever its form, behaves 

 optically like a single crystal of calcite. As regards the minute 

 structure of the calcareous spicules, Ebner's results differ totally 

 from those of Haeckel. Ebner was unable to find any residue 

 after dissolving the spicules with various acids, and his attempts 

 to demonstrate any such organic residue with stains gave negative 

 results. He explained the optical differences between the two 

 types of spicules, described by Haeckel as rich and poor in 

 spiculin respectively, by the fact that in the regular triradiates 

 the crystalline optic axis is vertical while in sagittal forms it is 

 inclined or even horizontal, in the facial aspect of the spicules. 

 Ebner pointed out further that the " browning " of the spicules 

 produced by heating is not due to the formation of carbon 

 through charring of organic substance, but is due to the disen- 

 gagement of fine bubbles of gas in the substance of the spicule 

 (compare Schmidt), making it opaque by transmitted light, milk- 

 white by reflected light ; with stronger heating the gas breaks up 

 the spicule with decrepitation. Ebner found, however, certain 

 differences between calcareous sponge-spicules and pure calcite, 

 and analysis showed the presence of magnesium, sodium, and 

 sulphates, as impurities mixed with the calcite comprising the 

 spicules. 



According to Ebner the axial filament of Haeckel is due to the 

 axial portion of the spicule having a different composition to the 

 peripheral portion, rendering the axis more easily attacked by 

 acids, by the action of heat, &c. ; but the difference between 

 axis and periphery is a gradual and quantitative, not a sharp 

 qualitative contrast. Ebner found the alleged stratification to 

 be present only in a few spicules, mostly very large forms. He 

 stvidied the stratification of the huge triradiates of Leucaltis 

 solida, and the large monaxons of Leucandra aspera and L. alci- 

 cornis, and found it also due to a special distribution of more, or 

 less, decomposable substances in different pai'ts of the spicule. 

 Thus the appearance of an axial filament and of stratification are 

 both due to a similar cause, namely, the periodic deposition of 

 more, or less, pure calcite in the building up of the spicule. 



Ebner concluded from his observations that the spicules of 

 Calcarea are mixed crystals consisting chiefly of calcite without 

 organic substance, but containing inorganic impurities (Na, Mg, S, 

 probably also water), and that the conditions of the mixture 

 differ at different periods of the growth and in different part sof 

 the spicule. 



Lendenfeld (1891), while quoting Ebner's results, reiterated 

 his former statements to the effect that " Each spicule-ray 



