1908.] SPICULES OF CALCAREOUS SPONGES. 669 



any sheath. Biitschli also (1908, p. 317) was unable to find any 

 remains of the sheath after dissolving the spicules with acid. 

 When, however, the acid was combined with the stain, both 

 filament and sheath were left intact and stained. Hence it is 

 probable that the disappearance of the sheath, when acid alone is 

 used, is due, not to the destruction of the sheath by acid, but to 

 its collapsing on the filament. This conclusion is supported by 

 the fact that when spicules stuck on with albumen are treated 

 with a combination of acetic acid and nigrosin, many of them 

 appear to contain unusually thick filaments, which are seen on 

 closer inspection to consist of the true axial filament together 

 with the collapsed sheath. Acetic acid would thus seem to have 

 a solvent or partially softening action upon the sheath. Biitschli 

 also found (1906, p. 317) that spicule-sheaths isolated by caustic 

 potash were dissolved completely by strong acetic acid, but were 

 preserved by very dilute acetic. Attempts to decalcify the 

 spicules with acid first and then to stain the sheath and axial 

 filament subsequently with nigrosin or indulin, were successful 

 when picric, nitric, or acetic acid were used, but not with hydro- 

 chloric acid. Various other stains were used without any effect 

 on the filament, for example carmine stains (borax- and alum- 

 carmine), Kernschwarz, indigo-cai'mine, &c. 



Appearance and Structure of the Axial Filament. 



The axial filament occupies a central position in the axis of the 

 spicule-ray. By focussing carefully the upper and lower surfaces 

 of the spicule-sheath in a spicule, decalcified and stained, it can 

 be clearly seen that the filament lies midway between the two 

 surfaces. In the optical transverse section of a ray, such as can 

 be easily obtained in the case of the gastral rays of the quadri- 

 radiates (fig. 4, PI. XXXV.), the filament appears as a black dot 

 occupying the centre of the ray, and can be traced up and down 

 the ray by focussing. The axial filament exhibits a certain 

 amount of tenacity and strength : this is shown by the fact that 

 when the decalcification proceeds irregularly in a spicule im- 

 bedded in albumen, detached fragments of calcite may be held 

 still for a time by the filament, until set free either by the 

 filament giving way under the strain, or by decalcification taking 

 place at the centre of the fragment round the filament, after 

 which the loose fragment rushes along inside the sheath of the 

 spicule. In spicules not imbedded in albumen, the sheath some- 

 times breaks across the ray, after decalcification and staining, 

 but the distal part of the sheath is held on by the filament,^ 

 which stands a great deal of bending and washing about without 

 bi'eaking aci-oss. In such preparations it is clearly seen that the 

 sheath is a very delicate structure, much less strong and resistant 

 than the actual filament. 



In the triradiate systems the axial filament appears to start 

 from the extreme tip of each ray of the spicule, and to be con- 



