( 16) 



As to the stnictnre of the spicules, Gray ('J 835) liad fdiind tliem 

 in Hiiahiii'iiia to consist of layers, which became conspicuous by 

 heating. These layers concentrically surround a "central canal", which 

 is filled out, as Kölliker (1864) has shown, by an organic mass, 

 llie axial rod. Claus (1868) found that the silica which directly 

 surrounds this central rod, is homogeneous; he called this homogeneous 

 cylinder the axial cylinder. x\ccording to Max Schultzp: (1860) the 

 longitudinal striae, which become conspicuous especially after heating, 

 are due to the fact that layers of silica alternate with \ery thin 

 layers of oi'ganic matter; the first are, after Schultze, isotropic, 

 the second anisotropic. The outer layer is generally found to be of 

 organic nature. 



In all these cases, the investigators described some special kind 

 of spicule ; naturally they have chosen very large spicules. Gray, 

 Claus and Max Schultzf studied the large rods of Hexactinellida, 

 such as H>/rtIonema and EiipIectcUa. It may be asked, how far 

 their results hold good for spicules of other sponges. 



Kölliker had already found that not in every case the axial 

 thread is conspicuous. Also it has not been possible to demonstrate 

 in every case alternating layers of spicopal and of organic matter, 

 not even where longitudinal lines are evident. There is a great 

 confusion with regard to the presence of a so-called spicular sheath. 

 Any accurate determination of the refractive power of different 

 spicules is likewise wanting. 



It seems, therefore, desirable to get some more information about 

 these subjects. Since F. E. Schulze (1904) studied the enormous 

 spicula of Moiiorhaphk, it appears useful to investigate, wliether 

 spicules from other groups agree with them as to their structure, 

 and their chemical and physical properties. 



We began our examination by the large styli of Tethya h/ncu- 

 rium. After bringing this point to some certainty, we haxe com- 

 pared the results with those obtained from other species. 



In the first place we tried to answer the question : Do the styli 

 of Tethya contain other organic elements than the central rod, either 

 as a sheath, or as layers between the spicopal, or as both of them. 



Of course, the method Avhich at first presents itself for the detec- 

 tion of organic matter, is the dissolution of the silica by means of 

 hydrofluoric acid. Foi-mer iuA-estigators who applied this reagent, have 

 omitted to give an accurate description of their experiments. Sollas 

 (1888 p. XLIX) put the isolated spicules into a drop of water and 

 added a drop of hydrofluoric acid. On doing this, one generally 

 sees that the silica is dissolved and that the central rod remains. 



