GEODIA MICROPORA. 



171 



170-330 fi in diameter, the centres of which are on an average 350 fx apart. 

 The pores in these sieves (Plate 37, figs. 8, 12) are very unequal, 30-130 [i wide, 

 and separated from each other by thin and narrow bands of dermal tissue. 

 Each pore-sieve forms the roof of a cavity from which a radial afferent cortical 

 canal, circular in transverse section and 50-200 ;t wide, arises. These affer- 

 ent canals penetrate the sterraster-armour layer and open out into a sub- 

 cortical cavity (Plate 37, figs. 1, 2, 3d). From the latter the narrow afferent 

 choanosomal canals originate. 



The flagellate chambers (Plate 37, fig. 14) appear to be sphaeroidal, de- 

 pressed in the direction of the oral axis, 20-26 p. broad, and 12-17 ji high. 



The choanosomal effercnts join to form wide canal-stems (Plate 37, fig. 2b) 

 which extend radially towards the efferent area of the surface. Some distance 

 below the cortex these canal-stems divide into branches leading up to the effer- 

 ent radial cortical canals. The latter are 200-500 ji wide and covered by sieves 

 (Plate 37, figs. 9, 13), the pores of which are larger and more equal than the 

 afferent pores, and usually 80-200 fi wide. 



The skeleton consists of large choanosomal amphioxes, small dermal rhabds, 

 orthoplagiotriaenes, mesoproclades, large oxyasters, large oxysphaerasters, 

 small strongylosphacrasters, and sterrasters. In the spicule-preparations I also 

 found chelotrops, which, however, are probably foreign to the sponge. In the 

 interior of the choanosome the large amphioxes are irregularly scattered. In its 

 distal part they, together with the rhabdomes of the orthoplagiotriaenes, form 

 radial bundles which abut vertically on the cortex (Plate 37, figs. 1, 2). The 

 cladomes of the orthoplagiotriaenes lie in the level of the inner limit of the 

 sterraster-armour layer. The rhabdomes of the mesoproclades are situated 

 radially, their acladomal ends are implanted in the sterraster-armour layer, their 

 cladomes protrude freely beyond the surface. The cladomal parts of these 

 spicules appear to form the bulk of the spicule-fur. A few large, slender, radial 

 amphioxes, however, are also found in it. Small dermal rhabds, mostly situated 

 obliquely, occur in considerable numbers in the subcortical layer, which appears 

 to be their place of origin (Plate 37, fig. 3c). From here they wander rapidly 

 — few only are found en route in the middle layer of the cortex — up to the 

 dermal layer, in which they take up their final position. Here they form tufts 

 implanted in the sterraster-armour layer. The distal parts of these tufts pro- 

 trude freely beyond the surface. These spicule-tufts occupy the tracts of 

 poreless tissue intervening between the pore-sieves. The tufts next the pore- 

 sieves are inclined towards their centre so as partly to shelter them. Most of 



