Sponge-fauna of Norway. 255 



fibres, which form the sphincters already mentioned. These 

 sphincters are well exposed by a tangential section of the 

 oscular tube- wall ; in such a section (PI. X. fig. 4) the wall 

 is seen to be divided into a number of polygonal areas, the 

 boundaries of which are marked by a few globate and acerate 

 spicules, while the greater part of the area of the polygon is 

 occupied by one of the sphinctral muscles, which, in carmine- 

 stained sections, have a deep red colour, strongly contrasting 

 with the uncoloured tissue of the polygonal boundary. 



In the cortex (PI. X. fig. 5) the oscular tube is lined by 

 epithelium bearing stellates, and overlying first a finely fibrous 

 layer, and then a thin stratum of vacuolated connective tissue, 

 which covers the globate-layer, here very much increased in 

 thickness, as also is the underlying cortical fibrous layer. 

 The roof of the oscular tube consists of a thin fibrous 

 layer, without globates, but traversed by acerate spicules and 

 covered by a layer of cylindro-stellates above and below. 



7. The Ciliated Chambers. — The spherical outline of these 

 chambers, which measure O'OOl inch in diameter, bears upon 

 its inner surface a number of small, round, highly refringent 

 nuclei with minute nucleoli, set at regular intervals from each 

 other ; but the outlines of complete cells cannot be made out, 

 any more than can the cilia. A sharply marked circular 

 aperture furnishes an abrupt passage from the interior of the 

 chamber to the incurrent canal, on which the chambers are 

 set, while the opening into the excurrent canal, on the other 

 hand, appears to be much more gradual and prolonged (PI. X. 

 fig. 6 A, and PI. XII. fig. 36) . 



8. The Mark. — The substance of the mark, independent of 

 the tissues which enter into the composition of the canal- 

 system, consists of finely granular sarcode, with large oval 

 nuclei, containing nucleoli (PI. XII. fig. 31) scattered through- 

 out it. It stains with carmine, but not so intensely as its 

 imbedded nuclei. The nuclei (PI. XII. fig. 29), which are 

 sometimes round as well as oval, have a well-marked double 

 contour, 0*0002 to - 0003 inch in diameter, and contain a 

 clear unstained space, within which is the deeply stained 

 round nucleolus 0*0001 inch in diameter. The mark-tissue 

 might be taken for a " syncytium," were it not that in some 

 cases distinct cells can be made out in it, having nuclei of 

 precisely the same characters as those just described, and 

 consisting of granular sarcode just like the ground-mass of the 

 mark. These cells (PI. XII. figs. 28, 32), 0*0008 inch in 

 diameter, have a very faint external contour; and one can 

 readily understand how, in a sponge not specially prepared 

 for histological examination, the borders of such cells would 



