( 23 ) 



It is evident, that if we are right in our conception of the spicule 

 sheath, other coats which sometimes are found surrounding spicules, 

 may not be called sheaths. 



F. E. ScHLj.zK describes in his last paper with his well known 

 accuracy such surroundings from the enormous needles of Mono- 

 rhaphis. We regret not to agree with him in calling this formation 

 "Scheide". As Schulze demonstrated it to be not only an investment 

 of each spiculum for itself, but also a means of joining different 

 spicules together, and as this is true for other spicules also, we propose 

 to call it periapt ^). Sollas (1880 p. 401 ^) described a similar coat 

 surrounding spicules from Isops pJdegraei. Bütschli found it in 

 Tethija h/ncurmm, a fact which we can confirm. The periapt is 

 composed of connective tissue with conspicuous tibrils and cells ; 

 it has therefore, nothing to do with the spicules as such and may 

 be left out of consideration here. Mutatis mutandis, the periapt 

 behaves to the spicule sheath as a perimysium to the sarcolemma. 



We have seen already that when the spicopal is dissolved first of all 

 the axial rod appears. The presence of such an organic thread in 

 macroscleres is no more doubted. Although we have no more 

 doubt that the axial thread is normally present, it cannot be denied 

 that in some cases it is wholly or partly absent. We consider 

 such cases, however, to be pathological. 



With regard to the shape of the rod Bütschjj (1901 p. 253) 

 writes: "eigenthünilich ist das Querschnittsbild des Fadens, das.... 

 stets deutlich dreieckig erscheint, gleichgültig ob der aussere Umriss 

 des Nadelquerschnitts selbst etwas dreiseitig oder ganz kreisrund ist. 

 Vielfach ist jedoch auf den Nadelquerschnilten zu erkemien, dass 

 der Querschnitt des Aclisenfadens sechseckig erscheint, indem die 

 Ecken des dreiseitigen Umrisses regelmiissig abgestumpft sind". 

 Consequently, the central thread in such cases, e. g. in Tethya, 

 would be a triangular rod and not a cylinder. Whereas already 

 Bo^vERBA^■K (1864) apparently had seen somediing like this in Geoilla, 

 F. E. Schulze (1904) arrived at the conclusion that the axial threads 

 in Hexactinellida are cylindric. In view of this contradiction we 

 thought it necessary to submit the axial thread of Tethya to a 

 cai'eful investigation. In order to judge about the shape of a thread 

 in transverse section, we followed tiie method of Bütschli by grinding 

 spicules in an agate morter. In the powder, pi-ocured in this way, 

 there is found always a sufficient quantity of |)articles of approximately 



^) Tr.piXTrru 1 bind together. 



2) Nol 1890; — appareully this is a misprint in Schulze (11)04 p. 204), — also 

 not p. 410—441 but 400—401. 



