196 



GEODIA MEDIA. 



massive form the pits are so close together and the pore-sieves covering theih so 

 extensive that the latter often come in direct contact with each other, so that 

 these depressions appear covered throughout by a nearly continuous pore-sieve. 

 In the digitate form such a junction of pore-sieves has not been observed. The 

 pores of these sieves lead into cavities up to 100 n wide, excavated in the dermal 

 tissue occupying the pits. The cavities of each pit join to form a radial cortical 

 canal about 250 pt wide, which penetrates the sterraster-armour and opens out 

 below into a subcortical cavity. These cavities are mostly 300-500 fi in radial 

 diameter. The proximal opening of each cortical canal is restricted by a ring- 

 shaped chonal sphincter, composed of an annular strand of contractile tissue 

 about 90 /i thick, which protrudes into the subcortical cavity. In the sections 

 studied nearly all these chonal sphincters were more or less dilated, the lumen of 

 some being as much as 140 fi wide. The cortical canals leading down from the 

 scattered pits of the digitate form are similar. As one radial cortical canal 

 belongs to each pit, these canals are in the depressions of the massive form about 

 1 mm., in the digitate form farther apart. 



The elevated borders surrounding the depressions of the massive form, and 

 parts of the convex portions of the surface of the digitate form appear to be 

 entirely destitute of pores. On other convex portions of the surface both of the 

 massive and the digitate form, pores, arranged in groups and forming pore- 

 sieves, are met with. These pore-sieves are much less extensive than the ones 

 covering the pits, never composed of a primary and secondary network, and 

 pierced by much larger holes. The latter are circular or oval and measure SO- 

 TO /L in diameter. The pores (holes) of each group lead into cavities, joining to 

 form a canal 200-300 fi wide, which traverses the cortex radially and is constricted 

 below by a chonal sphincter. In some places these radial cortical canals are 

 quite close together, their centres being only 1 mm. apart, in other places they 

 are much farther apart. 



In the basal part of the massive form very large canals, some as much as 

 6 mm. wide, are observed just below the cortex. In the digitate form I have not 

 met with any wide canals of this kind. In the choanosomal canals leading up 

 to the i)it-bearing parts of the surface transverse sphincter-membranes are spread 

 out at frequent intervals. 



The chonal sphincters of the cortical canals leading down from the pits 

 being wide open, the choanosomal canals leading up to them being traversed by 

 sphincter-membranes, and the pits themselves being situated on concave (de- 

 pressed) parts of the surface, one would suppose that the pores in the sieves 



