REPORT ON THE HEXACTINELLIDA. 23 



of the tube (though without the chambers) ; in Semperella, on the other hand, the cover 

 consists, in that portion which is spread over the afferent passages, simply of the uplifted 

 external skin, while the sieve plates extending over the oscular clefts appear as a 

 chamberless continuation of the entire wall of the tube. 



HISTOLOGY. 



Thoiigh the material placed at my disposal for investigation was not very well suited 

 for histological research, some facts of interest were established. It ought to be observed, 

 in the first place, that the histological structure is so uniform throughout the entire 

 group, that the modifications to be noted are hardly of au important character. 



As I pointed out in my communication On the Structure and Arrangement of the Soft 

 Parts in Euplectella aspergillum} the Hexactinellida exhibit, like all other Sponges, three 

 histological layers, viz., two distinct layers of epithelium, and an intermediate connective 

 tissue with various substances enclosed within it. As to the delicate single layer of 

 flat epithelum which covers the entire surface exposed to the water, I have not been able 

 to detect the contours of the cells, but simply their characteristic, small, clear, spherical 

 nuclei. These, and the small, shining, granule-like nucleolus, were distinctly recognis- 

 able on well-preserved portions of the surface when strongly stained with hsematoxylin 

 or picrocarmine. The nuclei are distributed with tolerable uniformity, and project a 

 little above the general surface of the cell, as is indisputably demonstrated on profile 

 view. They thus lie in the very outermost portion of the bounding surface. This can 

 be best seen on the dermal and gastral limiting membranes, but these epithelial nuclei 

 are also recognisable as slightly projecting elevations, on the netted strands of the sub- 

 dermal and subgastral trabecular spaces, and also on the trabeculse and membranes 

 extending between the ciliated chambers (PI. IV. fig. 8). 



A peculiar character, which differs not inconsiderably from the known relations of 

 other Sponge groups, is exhibited by the epithelium which lines the inner surface 

 of the chambers. One could not, of course, expect that in these epithelial cells, 

 structures so delicate as the flagellum and collar should be preserved. Although I have 

 taken every trouble, I have failed to detect these structures which are so constant in the 

 chamber-cells of all known Sponges. It cannot be doubted, however, that they were 

 really present. In my variously stained preparations, the chamber-cells appear as small 

 bodies, projecting for a variable distance into the lumen of the chamber, and provided 

 with small spherical nuclei and usually with a single shining nucleolus, which is 

 sometimes of a cylindrical, but usually of a kidney-shaped roundish form. The 

 remarkably regular disposition of these cells, and their connection Ijy flat lateral 

 bands, which extend over the membranous chamber- wall, are worth noting. Since 



' Trans. Eoy. Soc. Edin., vol. xxix., 1880. 



