162 



GEODIA OVIS. 



figs. 5b, 28; Plate 42, figs, la, b, 2a, 8a, b, 9a, b) composed of three layers: an 

 outer dermal layer, 100-200 p. thick ; a middle sterraster-armour layer, 150-300 [l 

 thick ; and an inconspicuous, inner fibrous layer, not sharply defined from the 

 choanosome. 



Canal-system. The parts of the surface which have lost their spicule-fur, 

 and which are consequently exposed to view, are occupied by pore-sieves (Plate 

 40, fig. 25). The pores in these sieves are oval or, more rarely, circular, and 

 measure 30 45 fi in maximum diameter. The strands of dermal tissue separat- 

 ing the pores are narrow, usually only 10-20 [x broad. The pores lead into cavi- 

 ties excavated in the dermal layer. From the latter radial canals, penetrating 

 the middle and inner layers of the cortex, arise. These radial canals are sur- 

 rounded by chonal sphincters which protrude inwards 300-400 n beyond the 

 sterraster-armour layer. The cylindrical chonal structures thus formed (Plate 

 42, figs. Id, 8) usually are 150-200 n in transverse diameter. The chonal 

 canal in the axis of these chones is cylindrical and usually open and up to 50 p. 

 wide. Some of these canals open out below with funnel-shaped extensions. 



The choaonosome is traversed by canals the widest of which are 2 mm. in 

 diameter. Many of them are provided with membranous sphincters at very 

 frequent intervals. The pore-sieves described above I take to be afferent. I 

 did not observe any that looked like efferents. To find these it would have been 

 necessary to remove the spicule-fur, and this I did not want to do as it would have 

 injured the unique and valuable specimen. From the general appearance of the 

 sponge I am inclined to conclude that the efferents are, like the afferents, cribri- 

 poral. 



The skeleton of the internal part of the choanosome consists of irregularly 

 disposed large amphioxes, a few styles (tylostyles), not very numerous asters, 

 mostly large thin-rayed oxyasters, some minute rhabds, and a few sterrasters. 

 The distal part of the choanosome is traversed by radial bundles of large mega- 

 scleres which abut vertically on the cortex. These bundles consist of numerous 

 large amphioxes, a few styles (tylostyles) with the rounded end situated distally, 

 and the rhabdomes of orthotriaenes, anatriaenes, mesoproclades, a few proclades, 

 and very few anamonaenes. The cladomes of most of the orthotriaenes and of 

 some of the anaclades are situated in the level of the lower limit of the 

 sterraster-armour layer. A few cladomes of these spicules and of the promeso- 

 clades (proclades), chiefly young forms, also occur at lower levels. Between 

 these spicule-bundles minute rhabds, large masses of asters, chiefly large thin- 

 rayed oxyasters, and a few sterrasters are met with. The asters are much more 

 numerous in this region than in the interior of the choanosome. 



