1908.] SPICULES OF CALCAREOUS SPONGES. 663 



concentric lamination seen in siliceous spicules, although both 

 siliceous and calcareous spicules break with a conchoidal fracture. 

 He points out that a central canal is very obvious in siliceous 

 spicules, " whereas in the calcareous one you can only fancy its 

 existence here and there." 



Haeckel (1872) described the structure of calcareous sponge- 

 spicules in a most detailed manner. The spicule-sheaths were 

 stated by him to be structureless envelopes arising as a thickness 

 of, and separation from, the ground-substance ; which, it must be 

 remembered, Haeckel regarded as a syncytial mass of protoplasm 

 or " sarcodine " formed by fusion of cells, and not as a secreted 

 gelatinous mesoglceal layer, as it is now universally held to be. 

 Haeckel contradicted Lieberkiihn's statement that the sheaths 

 were retractile. Each spicule or spicule-ray, according to Haeckel, 

 consists of a system of numerous very thin concentric laminae, 

 having the form of hollow cylindei"s or cones, surrounding a 

 common axis, a very fine central filament. The stratification and 

 the axial filament were stated to be universally present, though 

 the filament was much finer and more difficult to make out than 

 in siliceous spicules, and sometimes not visible unless the spicule 

 were moderately heated, whereby the axial filament was caused to 

 turn brown and then became visible by obliquely transmitted 

 light. The axial filament was stated to run up to the tip of the 

 spicule and become continuous with the " sarcodine," from which 

 it scarcely, if at all, differed in chemical nature. At the centre 

 of triradiates and quadriradiates a small spherical hollow was to 

 be seen, in which the axial filaments unite. 



The spicules were stated by Haeckel to consist of calcium 

 carbonate and water together with a varying amount of organic 

 matter, termed by him " spiculin," which was left behind as a 

 colourless and structureless residue when the spicule was dis- 

 solved by weak acids. The spiculin substance did not stain in 

 carmine, iodine, &c., and was dissolved by caustic alkalis ; its 

 presence was stated to be best demonstrated by moderately 

 heating the spicules. By the amount of spiculin present the 

 spicules could be placed in a series with two extreme types, the 

 one poor, the other rich in spiculin. The two extremes were 

 stated to be distinguishable at first sight under the microscope, 

 the spicules rich in spiculin appearing darker, more refractile than 

 those poor in spiculin, which were dull and pale in appeai-ance. 

 The phylogenetically older forms of spicules, that is to say, the 

 monaxons of simple form and the regular triradiates, were stated 

 to contain least spiculin ; the phylogenetically younger forms of 

 spicules, such as the sagittal triradiates, possessed most spiculin. 



Lendenfeld (1885) stated that spicules of calcareous sponges 

 consisted of carbonate of lime mixed with organic substance ; 

 by treatment with gold-potassium chloride the spicule was shown 

 to consist of " a great number of small prisms, parallel to one 

 another, radiating from the axis," which was " a cylindrical cord 

 of organic matter withoiit lime." The oldest part of the spicule 



