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extraordijuirily delicate; coiiseqiieiitly it is easily torn or shrunk. 

 Still, we are convinced that it exists, but the examination must be 

 carried out with the utmost care. In some preparations we found it 

 in the greater part of the objects. That it is not always observed 

 may partly be due to the treatment of the spicules with hydrochloric- 

 acid, partly by its being destroyed already during the life of the 

 sponge. The axial thread lb likewise not always visible, or at least 

 not over the whole length of the spiculum. On carefully dissolving the 

 spiculum it may be obser\'ed that, while the cylinder of silica grad- 

 ually diminishes its diameter, a very thin line shows the dimensions 

 it originally possessed. 



When the spicules are observed in Avater, the limits show them- 

 selves as rather broad black bands, as the refractive index of the 

 spicopal is considerably higher than that of water. When the dissol- 

 ving process goes on, the l)lack bands gradually approach each other, 

 and the thin line, the optical section of the spicule sheath, becomes 

 conspicuous. When the object is now studied in acid fuchsine, the 

 central thread stains intensely red as it is set free, and the sheath 

 becomes faintly reddish in the mean time. Organic layers, so called 

 layers of spiculine, such as can easily be demonstrated in the large 

 needles of Hexactinellida, are nowhere met with in Tethya. In some 

 cases we saw something Avliich resembled them, but in every case 

 we could ex[)lain the phenomenon by a folding of the sheath. Conse- 

 quently we conclude that layers of spiculine are absent in Tethya. 

 And we cannot agree with Minchin (^900), who says about Sjjicula 

 in general (I.e. p. 40): "the mineral matter is deposited round it 

 (viz. the axial thread) in concentric lamellae of colloid silica, alter- 

 nating with lamellae of organic nature". 



As to the s[)icule sheath, writers do not agree. What F. E. Scuulzk 

 calls "Spiculascheide" in his last publication (1904) is not homologous 

 with what we indicate with the name of sheath. That we notwith- 

 standing use this term has historical reasons, as it seems to us that 

 the word is originally used for formations belongijig to the spicule 

 itself, homologous to the product which is found in calcareous si)icules, 

 where its existence had been tirst demonstrated. In this sense 

 MiNcniN ajtplicd Ihu leriu, and he is tiie author of the newest and 

 best general treatise on Porifera. 



KöLLiKER (1864) may be regarded as the discoverer of the spicule 

 sheath. It seems to us beyond doubt what K()LLIkeii meant b}' it, 

 although we acknowledge that his opinion is not always expressed 

 with the utmost clearness, and Ihal from the beginning there had 

 existed some confusion of ideas about this organ. 



