38 THE VOYAGE OF H.M.S. CHALLENGER. 



of a distinction into primary fibres, taking their origin from the upper end of the stem 

 in order to assume a radial direction, as can be seen on pL xiii. of Esper's Pflan- 

 zenthiere, ii., and into secondary fibres uniting the primary ones with one another both 

 perpendicularly and parallel to the surfaces of the sponge. Both the surfaces are uneven, 

 and this is due to tlie circumstance that the secondary fibres, which are nothing but 

 lateral branches of the primary ones, interlacing amongst themselves, form prominent 

 irregular networks. The last-named are most fully developed near the upper end of the 

 stem, growing gradually lower towards the edge of the leaf -like part of the animal. The 

 external surface of this stem is comparatively smooth ; its skeleton admits of no distinction 

 into primary and secondary fibres, the whole presenting a very compact and solid network, 

 with meshes of smaller diameter than those of the leaf-like extension (PL II. fig. 3). 

 The fibres themselves, as is weU known, are heterogeneous and, on the whole, to be classed 

 as thick walled. The microscopical structure of the central pith-substance docs not differ 

 from that of Aphjsina, as described by F. E. Schulze ^ in the case of Aj^lysina aerojihoha, 

 nor do the walls surrounding this central differentiation, and presenting, as is always the 

 case, many layers of concentrically disposed horny laminae, with the sole distinction that 

 between these latter true cells are to be found in abundance. As to the histological 

 properties of these interlamiuar elements, I refer the reader to Dr. Flemming's ° paper on 

 lanthella, for I can confirm all his statements except as to the ends of the fibres being, as 

 he suggests, devoid of cells. For my own part, I could always discern them even on the 

 youngest fibres, viz., on the fibres with only a very thin horny envelope, and am able 

 to state that they are absent only at the ends of developing fibres, represented by 

 pith-substance surrounded immediately by spongoblasts. That the ceUs enclosed in the 

 walls are nothing but transformed spongoblasts is beyond doubt, but of course this will 

 be actually proved only when direct observations on the actual process of development of 

 the fibres in question have been made. For myself, amongst the elongated sj)ongoblasts, as 

 I have drawn them on PI. II. fig. 5, I have very often seen cells of a more massive and also 

 compressed form, and I believe these to be intermediate stages between the typical 

 spongoblasts and the interlamiuar cells. But I have not figured them, because their 

 deviating appearance may yet be ascribed to the influence of the preserving fluid. 

 Around the "mantle of spongoblasts" of young fibres I always found aggregations of 

 mesodermic cells, though by no means always disposed parallel to the developing fibre 

 as drawn and described by v. Lendenfeld ^ in his Bendrilla rosea and Dcnclrilla aerophoha. 

 Around old fibres I have missed them completely. According to Flemming, the colour of 

 the fibres of lanthella hasta, so fer as the horny substance of then." laminae is concerned, is 

 yellow, that of the enclosed cells deep violet. I have found in most cases the colour 

 of the laminae also to be violet. I am, however, inclined to ascribe this merely to the 



' Zeitschr.f. wiss. Zool, Bd. xxx. p. 401, pi. .\xii. tig. 11. ^ Wiirshurg Verhandl., N. F., Bd. ii. 



3 Zeitschr. f. wiss. Zool, Bd. xxxviii. p. 286 ; pi. xiii. figs. 25, 28, 29. 



