1908.] SPICULES OF CALCAREOUS SPONGES. 665 



consists of a thin, somewhat turbid (triib) axial thread, enveloped 

 by the hyaline (glashell) spicule-substance. In the latter a con- 

 centric sti'atification round the axial filament is to be recognised " 

 (1. c. p. 369). 



Minchin (1898, p. 569) stated that the spicules of Clathrina 

 coriacea, if treated with picro-nigrosin (saturated solution of picric 

 acid in water, 9 volumes ; 1 "/o nigrosin in water 1 vol.), showed 

 the sheath and the axial filament, left behind after solution of the 

 calcareous matter, and stained blue by the nigi-osin. 



Biitschli (1901) published elaborate investigations upon the 

 minute structure of siliceous and calcareous sponge-spicules, 

 taking the large monaxons of Leucandra as2)era as examples of 

 calcareous spicules. His results were, in the main, confirmatory 

 of Ebner's. He found that moderate heating produced a finely 

 alveolar structure in the spicule. No axial canal or filament was 

 found to be present ; the axial thread, sometimes visible after 

 moderate heating, was stated to be due to a modification of the 

 calcareous substance, which at the central part of the spicule was 

 distinguished from the remaining part by greater refractility and 

 by being moi'e easily attacked by acids. No trace of an axial 

 filament or sheath was found when spicules were dissolved by 

 acetic acid. The stratification was ascribed to the concentric 

 arrangement of the minute alveoli composing the spicule. 



Unlike Ebner, Biitschli found small quantities of organic 

 substance left when the spicules were dissolved with hydrochloric 

 acid. The spicule-sheaths were found to be not purely organic 

 in nature but to contain a certain amount of inorganic matter, 

 probably calcium carbonate. 



Maas (1904) and Weinschenk (1905) agree in assuming that 

 the calcite of the spicule must be combined with finely divided 

 organic substance. Maas explains the solvent action of caustic 

 alkalis on the spicules by the supposition that the caustic attacks 

 a substance holding togethei- the constituent particles, and so pro- 

 diices a disaggregation of calcareous elements which were formerly 

 crystallographically orientated. He considers the phenomena 

 seen on heating to be explicable also on the assumption of a 

 finely distributed organic material in the spicule ; and he showed 

 that when the sponge is grown in water deprived of CaCOg the 

 organic substratum alone of the spicule is secreted. Weinschenk 

 dwells on the difierences between calcareous sponge-spicules and 

 pvire calcite, and considers these differences, and also the peculiar 

 form of the spicules, explicable only by the presence of a fine 

 organic tissue in the spicules. 



Biitschli (1906) controverts the statements of Weinschenk 

 upon certain points which are of secondary importance for the 

 subject of this memoir, and maintains his former position. He 

 denies that the opacity and decrepitation produced by heating is 

 due to the presence of organic matter in the spicules. 



It is seen from the foregoing that opinions are greatly divided 

 upon the question of the structure and composition of calcareous 



