256 Mr. W. J. Sollas on the 



become altogether ^indistinguishable in the majority of cases, 

 and so, by a deceitful appearance of confluence, give rise to 

 the notion of a syncytium. Connective tissue like that of the 

 medusoid disk is not discoverable in this sponge ; in Thenea 

 Wallichu, Wright, however, the greater mass of the mark 

 consists of it. 



9. The Spicules. — The long-shafted spicules are enveloped 

 in a sheath which somewhat resembles the epithelial mem- 

 brane, and are accompanied by longitudinally arranged fibres 

 like those of the cortex ; they are also frequently closely sur- 

 rounded by concentric fibres of a very simple appearance, 

 consisting merely of thin flat fusiform hyaline strips with a 

 small round central granule or nucleus. 



The globates of the cortex are all full-grown forms ; but 

 those dispersed through the mark are to be met with in all 

 stages of development. In their earliest state they consist of 

 minute trichites, radiately arranged to form a sphere, the centre 

 of which is either empty or occupied by some transparent 

 substance like that of the axial thread of a long-shafted spicule. 

 The outer ends of the trichites penetrate a thick double-con- 

 toured cell-wall, which is at first transparent and almost 

 colourless (PI. XII. fig. 37). On one side of this cell-wall is 

 imbedded an oval nucleus, which strikingly resembles the 

 nuclei of the mark-cells. With growth a deposit of silica is 

 formed about the inner ends of the trichites, cementing them 

 together into a transparent siliceous globule ; the outer diver- 

 ging ends remain unenveloped, and are easily detached from the 

 central sphere. A hilum is for some time absent ; but pre- 

 sently the growth of the trichites beneath the nucleus becomes 

 slow compared with that outside it (PI. XI. fig. 18), and 

 as a result a conical cavity is left under the nucleus and 

 forms the hilum of the adult spicule. The nucleus, when 

 viewed face on, appears to rest, like a biconvex lens, over 

 the upper end of the hilum ; but a lateral view presents it 

 as completely filling the cavity of the hilum. The cell- 

 wall enlarges with the growth of the globate, and very early 

 acquires a very granular appearance and a deep grey colour ; 

 it then stains deeply with carmine. Probably the preceding- 

 statement should in one point be reversed, and we should say 

 that the trichites increase in length with the growth of the 

 cell-wall. Finally the trichites become thicker and acquire 

 rounded conical ends, which at length assume the character- 

 istic adult form. 



It is singular that no immature forms are met with in the 

 cortex ; and this leads one to infer that the fully-grown globate 

 travels in some manner unknown from the mark to the lower 



