IIG GEODIA AGASSIZII. 



ones (Plate 27, fig. 1) merely a thin dermal membrane. In other adult speci- 

 mens (Plate 27, fig. 2a) it is 150-200 /i, rarely as much as 230 /«, thick and com- 

 posed of loose tissue, containing small amphioxes and strongylosphaerasters, but 

 no sterrasters or oxyasters. The sterraster-armour layer (Plate 27, figs, lb, 2b; 

 Plate 32, figs. 8a, 11a, 12a) is in the young specimen from Station 4228 350- 

 400 fi, in the adult specimens usually about 800 fi thick. The inner, fibrous 

 layer is free from sterrasters and quite thin. The limit between the dermal 

 layer and the sterraster-armour layer is very clearly defined, the limit between 

 the latter and the inner, fibrous layer is somewhat indistinct. 



Canal-system. The areas of the large efferent holes in the sterraster- 

 armour are, in all sufficiently well-preserved specimens, covered by a dermal 

 membrane perforated by numerous small afferent pores. These lead into 

 9 systems of canals traversing the dermal layer and converging to points lying 

 in the level of the limit between this layer and the sterraster-armour layer. 

 Here the canals of each system join to form a radial tube, surrounded by a 

 chonal sphincter, which occupies one of the small afferent holes in the 

 sterraster-annour. The afferent cortical canals are in all the sections ex- 

 amined very narrow, or quite closed. Below the sterraster-armour layer these 

 canals open out into subcortical cavities (Plate 27, fig. 2c) which are higher 

 than broad and often attain a radial dimension (height) of 1 mm. The chonal 

 sphincters do not protrude into these cavities. From the majority of these 

 subcortical cavities narrow afferent canals lead down to the adjacent flagellate 

 chambers. Some of the subcortical cavities join below to form large afferent 

 canals (Plate 27, figs. Id, 2d) 1-2 mm. wide, which, repeatedly ramifying, supply 

 the more distant flagellate chambers. The flagellate chambers are spherical 

 and measure 27-35 ft, usually about 30 /t, in diameter. The efferent canals aris- 

 ing from them join to form large tubes (Plate 27, fig. le; Plate 32, fig. 5a), 1 

 mm. or more in diameter, which extend towards the areas of the large efferent 

 holes in the sterraster-armour layer. The afferent canals are not separated 

 from the chamber-bearing choanosomal tissue by special mantles and have 

 smooth surfaces. The elTerent canal-stems on the other hand are, particularly 

 in their wider distal parts (Plate 27, fig. le; Plate 32, fig. 5a), enclosed in 

 sheaths, about 500 /« thick, free from flagellate chambers, and greatly con- 

 stricted at very frequent intervals by transverse sphincter-membranes, pro- 

 truding far into their interior. Distally these efferent canal-stems divide 

 into branches which lead up to the cortex. From the summits of these 

 branches arise radial cortical canals (Plate 26, figs. 13, 14a, 15a), surrounded 



