102 GEODIA OVIS. 



figs. 5b, 2S: Plate 42. figs, la, b, 2a, Sa, b, 9a, b) composed of three laj-ers: an 

 outer dermal layer, 100 200 /< thick; a middle sterraster-armour layer, 150-300 ji 

 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 fx 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 ft beyond the 

 0sterraster-armour layer. The cylindrical chonal stmctures thus formed (Plate 

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

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

 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 ver}^ 

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

 did not observe any that looked hke 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 

 injiu'cd the unique and valuable specimen. From the general appearance of the 

 sjjonge I am inclined to conclude that the efferents are, like the afTerents, cribri- 

 poral. 



The skeleton of the internal j^art of the choanosome consists of irregularly 

 disposed large amphioxes, a few styles (tylostyles), not very numerous astci-s, 

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

 The distal ixirt of the choanosome is traver-sed by radial l)undles of large mega- 

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

 large amphioxes, a few styles (t}-lostyles) 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 tlie lower limit of the 

 sterraster-armour laj-er. A few cladomes of these sjjiculcs and of the promeso- 

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

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

 ra3-ed oxyastei-s, and a few sterrastei-s are met with. The asters are much more 

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



