GEODIA ATAXASTRA. gj 



separated by rather broad strands of dermal tissue. The pores of each group 

 lead into a system of lacunose, subdermal canals, which converge and unite 

 below the centre of the pore-group to form an afferent radial canal (Plate 44, 

 fig. 26b). The subdermal afferent canals are wide, separated only by relatively 

 thin walls, vertical to the surface of the sponge, which radiate from a common 

 centre. These walls attach the pore-sieves to the sterraster-armour and give to 

 them, when viewed en face, the stellate appearance referred to above (Plate 43, 

 fig. 26). The radial cortical afferent canals are 120-230 ju wide and circular in 

 transverse (paratangential) section. Their centres are 300-600 n apart. 



Below the sterraster-armour layer numerous small cavities, measuring on 

 an average about 100 /« in radial diameter, are met with (Plate 44, fig. 26). 

 The radial cortical canals open out into these cavities, and from them the choano- 

 somal afferents take their rise. The flagellate chambers (Plate 44, fig. 24) are 

 spherical and measure 15-20 fi in diameter. In the interior of the choanosome 

 large canals, some 1 mm. and more wide, are obsei-ved (Plate 43, fig. 25). 



The efferent cortical canals, which are confined to the efferent areas referred 

 to above, also have a circular transverse section. They are 200-300 [i wide and 

 their centres mostly 600 /z-1 mm. apart. In places, particularly towards the 

 margins of the areas occupied by them, they are more distant. The outer 

 openings of these canals are covered by nets of dermal strands, only 10-20 n 

 broad. The meshes of these nets are oval or polygonal, 70-200 /x broad, and up 

 to 400 i-L long (Plate 43, fig. 28). Thus, when the efferent area is viewed en face 

 one sees onlj^ a few strands or a loose net work of strands spread out over the 

 entrances of the efferent cortical canals. 



Skeleton. Strands of rhabds, extending obliquely or paratangentially, 

 traverse the deeper parts of the choanosome. The rhabds composing them are 

 chiefly amphioxes, but amphistrongyles, styles, and angularly bent or irregularly 

 branched amphiox-derivates also occur in them in small numbers. In var. 

 latana, the amphistrongyles and stj-les are relatively much more numerous 

 than in var. angiistana. In the distal part of the choanosome similar rhabds 

 and the rhabdomes of orthoplagiotriaenes, anatriaenes, and mesoproclades, 

 chiefly mesoprotriaenes, form radial bundles. The rhabdomes of some of the 

 orthoplagiotriaenes are reduced in length and rounded at the end. Ortho- 

 plagiotriaenes with such rhabdomes have been cliiefly observed below the thin 

 part of the cortex of a specimen of var. angustana, mentioned above. The 

 cladomes of nearly all the orthoplagiotriaenes and of a large number of the 

 anatriaenes lie in the level of the lower limit of the sterraster-armour. A few 



