440 G. C. J. Vosmaer 



together also (fig. 22) . Let ns call for a moment these latter ones pri- 

 mary fibres. those Coming oif from them secondaiy ones. The primavy 

 fibres go for the greater part in a longitudinal direction ; but afterwards 

 they bent out toward the periphery ; the result is that the outer, thinner, 

 younger parts go in a nearly radial direction. During their course they 

 send off secondary branches, which connect the primary branches. Thus 

 on transverse sections a network of fibres is seen, the radial ones of 

 which consist mainly of the secondary fibres, but partly of the beut-out 

 extremities of the primary ones. Those fibres that go in a concentric di- 

 rection consist on the contrary only of secondary branches. Now in obser- 

 ving a great quantity of sections, transverse, longitudinal and tangential 

 a certain regularity is evident. Themeshes are for the greater part square. 

 As a rule thesix fibres forming the longitudinal, concen- 

 tric and radial Systems, meet at approximately right angles. 

 Among the siliceous Sponges there is a large family characterised 

 by the hexactinellid structure of the skeleton. Most of them have a solid 

 skeleton, the Clements of which are sex-radiate spicules, besides this 

 however they possess free sex-radiate spicules. Among the Sponges that 

 are destitute of self-produced siliceous Clements Fritz Müller found the 

 first sex-radiate horny spicules. Velinea is an interesting example of a 

 Sponge possessing a solid hexactinellid horny skeleton. According to 

 F. E. Schulze 1 the whole skeleton-fibre oi Euspongia is a secretion 

 of modified cells of the connective tissue. The axis is the first pro- 

 duction, the concentrical layers of »spongin« are formed afterwards, but 

 both are secretions of the spongoblasts. According to Marshall and 

 Schmidt the fibres take their origin from simply modified »sarcodine«. 

 In his studies ou Hexactinellids Marshall says that in one case the net- 

 work of sarcodine (»Sarcodine-gitterwerk«) is enveloped by horny ma- 

 terial, in another case by silica (Hexactinellids) . The first we find in 

 Hornsponges; as examples Marshall mentioued Z2f/f«r?V/ Duch . et Mich, 

 and Aplysina 0. S. Had he known our Sponge, he would probably he 

 stili more convinced of his hypothesis: we now know a Hornspouge 

 with sex-radiate free spicules and another with a rather regulär sex- 

 radiate skeleton. Marsh all's hypothesis seems to be no more than that. 

 Whether there is really a dose relation betweeu the two mentioned 

 groups is for the moment not yet made out, because we do not know 

 how the skeleton developes. Marshall's suppositiouis iugenious, but not 

 entirely true, because he did not know of the existence of spongoblasts. 



1 F. E, ScHiTLZE, Zeitschr. f. wiss. Zoo!. XXXII, p. 635. 



