SIDONOPS OXYASTRA. 



41 



A cortex (Plate 7, figs, la, 2a, 6a), composed of a thin outer dermal layer, 

 a thick middle sterraster-armour layer, and a thin inner fibrous layer, is developed 

 superficially. The whole cortex is, under most parts of the surface, 700-750 n, 

 in some parts of the efferent areas up to 1.6 mm. thick. As such thicknesses of 

 the cortex have been observed only between widely open efferent cortical canals 

 they may be produced by the dilatation of the latter. The dermal layer is in 

 the afferent areas on an average about 40 n thick and excavated by systems of 

 subdermal canals. In the efferent areas it is on an average about 60 n thick, 

 solid, and occupied by numerous paratangentially disposed spindle-cells. The 

 sterraster-armour layer is in the afferent areas about 600 n thick and every- 

 where, except in the walls of the cortical canals, occupied by dense masses of 

 sterrasters. In parts of them the portions of the middle layer free from sterr- 

 asters around the cortical canals are rather extensive and considerably widened 

 distally (Plate 7, fig. 6). The inner fibrous layer is 35-70 [i thick and occupied 

 by paratangential, somewhat undulating fibres, staining strongly with aniline- 

 blue. 



The choanosome is traversed by strands composed of large and conspicuous 

 elongated cells (Plate 6, figs. 1, 2). These strands are 60-90 p. broad. The cells 

 composing them are arranged rather irregularly, but on the whole distinctly 

 longitudinally. They are mostly spindle shaped, 20-30 long and 4-7 /( thick. 

 Here and there (Plate 6, fig. 1, to the left below) thicker, more oval elements, 

 measuring 20 X 11 /<, are observed in the strands. The plasma of these cells is 

 occupied by large granules, staining strongly with haematoxylin. 



Canal-system. The afferent areas of the surface are occupied by sieve-like 

 pore-groups 0.5-1 mm. in diameter (Plate 8, fig. 13). These pore-sieves he 

 close together, being separated only by narrow poreless tracts. The afferent 

 pores themselves (Plate 8, fig. 15) are in the preparations, probably in conse- 

 quence of post mortem shrinkage, somewhat irregular in shape, 40-120 n wide, 

 and separated by dermal bands of varying breadth. The broader bands, which 

 form a sort of primary network, are thick, reach down to the middle layer of 

 the cortex, and contain asters and dermal rhabds. The narrower bands which 

 connect these primaries, are quite thin and contain only a few small asters or 

 no spicules at all (Plate 8, fig. 15). The pores of each group (pore-sieve) lead 

 into a system of subdermal cavities, which converge and unite to form a radial 

 canal. These radial afferent cortical canals, which penetrate the sterraster- 

 armour layer, are circular in transverse section and about 45 /j. wide. They are 

 distributed somewhat regularly over the afferent areas, their centres being 0.7-1 

 mm. apart. 



