GEODIA VARIOSPICULOSA. 57 



or ratlially. Strands of such fibres extend from this layer far down into the 

 choanosome. 



Canal-system. The pits on the surface are covered by pore-sieves. These 

 are so numerous and so close together that they join to form extensive con- 

 tinuous pore-areas. The pores are generally broad-oval, 20-75 n long and 15- 

 55 fi broad. They seem to be larger in var. intermedia than in var. micraster. 

 The strands of tissue separating them vary very much in width. Some are so 

 narrow as to appear as slender threads, w'hile others are as broad as or even 

 broader than the pores. On examining pore-sieves, removed by a paratangential 

 section in transmitted light, one clearly sees that these sieves are composed of 

 stout, primary, non transparent bars, the interstices between which are occupied 

 by secondary nets of thin, transparent strands (Plate 18, fig. 9). The stout bars 

 of the primary network usually exhibit a somewhat radial arrangement round 

 the centre of the pore-sieve (pit). Below these sieves rather extensive 

 cavities occur into which the pores lead. These cavities join under the centre 

 of each pore-sieve, that is, in the centre of each pit, to form a radial canal which 

 penetrates the cortex and either opens out below into a subcortical cavity 

 (Plate 18, fig. 21) or is continued as a narrow, usually tortuous canal leading 

 down into the choanosome (Plate 18, fig. 26). The proximal third or half of 

 each radial cortical canal is surrounded by a stout chonal sphincter, which 

 does not extend proximally beyond the lower limit of the sterraster-armour 

 -layer. In the sections examined the radial cortical canals are constricted and 

 their proximal portions, which pass through the chonal sphincter, are often 

 quite closed. In the interior some large canals, in var. intermedia up to 0.8, in 

 var. micraster up to 2 mm. wide, are observed. In the latter a cavity, about 

 1 mm. wide, surrounded by sterrasters, was observed 2 mm. below the surface. 



Skeleton. Rather loose strands of amphioxes and a few tylostyles and 

 styles traverse the inner part of the choanosome. These internal spicule-strands 

 are not arranged in a regularly radial manner. Many are very oblique, and 

 once I saw one extending paratangentially about 6 mm. below the surface. 

 Distally the megascleres form bundles which penetrate the inner layer of the 

 cortex and terminate at, or a little above, the lower limit of the sterraster-armour. 

 These distal spicule-bundles (Plate 18, fig. 8b) are vertical or oblique to the 

 surface and contain, besides the large amphioxes and occasional monactmes 

 found in the interior, numerous rhabdomes of orthoplagiotriaenes and some 

 rhabdomes of dichotriaenes, large anatriaenes, and mesoclades, chiefly mesopro- 

 triaenes. The cladomes of most of the orthoplagiotriaenes and dichotriaenes 



