Sponge-fauna of Norway. 155 



the external ends of the spicular columns they appear to 

 run at right angles to them, and appear to unite with them 

 bj their frayed and expanded ends. 



At its base the spicular column expands by a thickening 

 of the mesoderm as it extends along with the ectoderm over 

 the upper face of the fibrous layer of the cortex, to form the 

 floor of the subdermal cavities. Below this superficial 

 stratum the fibrous layer consists of similar fusiform fibres to 

 those of other parts of the mesoderm, but surrounded by a 

 more condensed layer of matrix ; they are about yio" i'^ch 

 loiigj H) 6 u iiich wide, highly refractive, faintly bluish, granular, 

 with an oval vesicular nucleus and round refractive nucleolus ; 

 the surrounding mesoderm, the walls of these cells, is clear, 

 colouidess, and sometimes slightly fibrillated. The inner 

 face of the fibrous layer is coated with an adherent granular 

 mark. 



The fibrous layer is traversed by the smaller fusiform 

 acerate spicules represented by Bowerbank (Brit. Spong. 

 vol. i. fig. 362), which run through the fibres like stakes 

 through wattlework. In Bowerbank's figure these spicules 

 all lie parallel to each other, at right angles to the general 

 direction of the fibrous layer; but in none of my slices is 

 such an arrangement to be seen ; the majority of the spicules 

 run obliquely through the fibrous layer, sloping convergently 

 towards the spicular columns at their base, and midway be- 

 tween crossing each other obliquely in different directions, 

 some running at right angles to the fibres. The fibres of the 

 fibrous layer lie concentric with the surface of the sjjonge, 

 running in winding curves orientated in all directions, like 

 the similar fibres in the cortex of Stelletta Normani (Ann. 

 & Mag. Nat. Hist. 1880, vol. v. pi. vi. fig. 3). The deeper 

 half of the fibrous layer has a duskier, darker tint than the 

 more superficial, and takes a deeper stain with carmine. As 

 to its function, it is probably a fibrous connective tissue, as I 

 previously asserted of the corresponding layer of Geodia 

 Barretti (Ann. & Mag. Ntit. Hist. vol. v. p. 253). 



The fibrous layer becomes still more modified in the same 

 direction immediately about the incurrent canals, a little 

 below the middle of their course through it ; here the fusi- 

 form axial threads are more closely approximated, the inter- 

 vening tissue is of a darker grey, and the stain with carmine 

 strikingly marked ; the arrangement of the fibres is for the 

 most part very regularly concentric ; but a few are radiately 

 disposed. With this change in appearance there is a change 

 of function, and the fibres form a true muscular sphincter, 

 which is to be observed in the prepared slices in all stages of 



