GEODINELLA ROBUSTA. 



207 



narrow, or altogether closed, and separated from the nearest sterrasters by a 

 thick intervening layer of soft tissue. The canals passing through the large 

 perforations of the sterraster-armour are usually dilated and open out on the 

 surface of the sponge with a wide aperture. These apertures are the vents, 

 which as stated above, crown the summits of some of the elevations. The 

 canals passing through the smaller perforations are usually contracted. On 

 account of the hardness of the cortex and also because I wished to preserve such 

 valuable material, I have not been able to ascertain exactly how these narrow 

 radial canals open out on the surface. So far as I could make out they divide 

 distally into several oblique nearly paratangential canals, which lead to small 

 groups of round pores, 50-80 /« wide, above the perforation of the armour through 

 which the radial canal passes. Below, these radial canals are provided with 

 chones composed of brownish tissue. These chones are about 200 /i broad and 

 protrude to a distance of 400 /« into the choanosome. 



The choanosome (Plate l, figs. 21b, 24b; Plate 2, fig. 6b) is remarkably 

 soft and traversed by numerous wide canals up to 1.5 mm. in diameter. The 

 widest of these canals usually extend paratangentially just below the surface 

 (Plate 1, figs. 21, 24). From the canal-walls numerous low transverse ridges 

 protrude into the interior, partially dividing the canal-lumina into rows of 

 chambers. Such ridges occur not only in the wide, but also in the narrow canals. 

 The flagellate chambers (Plate 2, figs. 4b, 11a) are longitudinally compressed, 

 the length of the main axis, passing through the chamber-mouth, usually being 

 only from two thirds to three fourths of the length of the transverse diameter. 

 In the specimen of var. carolae from Charlotte Sound, which is better preserved 

 than the others, the flagellate chambers are 20-35 ft long and 30-45 p. broad.^ 



In the characters of their canal-system these sponges appear on the whole 

 to approach rather closely the species of Sidonops. 



// istological structure. Many of the flagellate chambers have lost their collar- 

 cells altogether, in others collar-cells are present, but they are never numerous 

 (Plate 2, figs. 4, 11). Whether this sparseness of the collar-cells is due to some 

 having dropped off post mortem, or whether it is a natural condition of the living 

 sponge, I cannot say. "SoUas's membranes" about 9 pt distant from the 

 chamber-walls are frequently observed. The nuclei of the collar-cells are about 

 2 p. m diameter. The dermal membrane is thin and composed of stout, spindle- 

 shaped elements, extending paratangentially. The proximal part of the corti- 

 cal tissue, just below the sterraster-armour, contains coarse paratangential 

 fibres and granular oval or spherical cells, 15-30 p in diameter. In the walls 



